diff --git a/packages/babel-helpers/src/helpers.js b/packages/babel-helpers/src/helpers.js
index 23416e0022..8fb04a8953 100644
--- a/packages/babel-helpers/src/helpers.js
+++ b/packages/babel-helpers/src/helpers.js
@@ -1040,9 +1040,7 @@ helpers.initializerWarningHelper = helper("7.0.0-beta.0")`
     export default function _initializerWarningHelper(descriptor, context){
         throw new Error(
           'Decorating class property failed. Please ensure that ' +
-          'proposal-class-properties is enabled and set to use loose mode. ' +
-          'To use proposal-class-properties in spec mode with decorators, wait for ' +
-          'the next major version of decorators in stage 2.'
+          'proposal-class-properties is enabled and runs after the decorators transform.'
         );
     }
 `;
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/local-define-property/input.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/local-define-property/input.js
new file mode 100644
index 0000000000..f939e63b3b
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/local-define-property/input.js
@@ -0,0 +1,12 @@
+function dec() {}
+
+// Create a local function binding so babel has to change the name of the helper
+function _defineProperty() {}
+
+class A {
+  @dec a;
+
+  @dec b = 123;
+
+  c = 456;
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/local-define-property/options.json b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/local-define-property/options.json
new file mode 100644
index 0000000000..6f03cd7aad
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/local-define-property/options.json
@@ -0,0 +1,7 @@
+{
+  "plugins": [
+    ["proposal-decorators", { "legacy": true }],
+    ["proposal-class-properties"],
+    "transform-classes"
+  ]
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/local-define-property/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/local-define-property/output.js
new file mode 100644
index 0000000000..641307b44d
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/local-define-property/output.js
@@ -0,0 +1,40 @@
+var _class, _descriptor, _descriptor2, _temp;
+
+function _initializerDefineProperty(target, property, descriptor, context) { if (!descriptor) return; Object.defineProperty(target, property, { enumerable: descriptor.enumerable, configurable: descriptor.configurable, writable: descriptor.writable, value: descriptor.initializer ? descriptor.initializer.call(context) : void 0 }); }
+
+function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
+
+function _defineProperty2(obj, key, value) { if (key in obj) { Object.defineProperty(obj, key, { value: value, enumerable: true, configurable: true, writable: true }); } else { obj[key] = value; } return obj; }
+
+function _applyDecoratedDescriptor(target, property, decorators, descriptor, context) { var desc = {}; Object.keys(descriptor).forEach(function (key) { desc[key] = descriptor[key]; }); desc.enumerable = !!desc.enumerable; desc.configurable = !!desc.configurable; if ('value' in desc || desc.initializer) { desc.writable = true; } desc = decorators.slice().reverse().reduce(function (desc, decorator) { return decorator(target, property, desc) || desc; }, desc); if (context && desc.initializer !== void 0) { desc.value = desc.initializer ? desc.initializer.call(context) : void 0; desc.initializer = undefined; } if (desc.initializer === void 0) { Object.defineProperty(target, property, desc); desc = null; } return desc; }
+
+function _initializerWarningHelper(descriptor, context) { throw new Error('Decorating class property failed. Please ensure that ' + 'proposal-class-properties is enabled and runs after the decorators transform.'); }
+
+function dec() {} // Create a local function binding so babel has to change the name of the helper
+
+
+function _defineProperty() {}
+
+let A = (_class = (_temp = function A() {
+  "use strict";
+
+  _classCallCheck(this, A);
+
+  _initializerDefineProperty(this, "a", _descriptor, this);
+
+  _initializerDefineProperty(this, "b", _descriptor2, this);
+
+  _defineProperty2(this, "c", 456);
+}, _temp), (_descriptor = _applyDecoratedDescriptor(_class.prototype, "a", [dec], {
+  configurable: true,
+  enumerable: true,
+  writable: true,
+  initializer: null
+}), _descriptor2 = _applyDecoratedDescriptor(_class.prototype, "b", [dec], {
+  configurable: true,
+  enumerable: true,
+  writable: true,
+  initializer: function () {
+    return 123;
+  }
+})), _class);
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/loose/input.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/loose/input.js
new file mode 100644
index 0000000000..92298f2eb4
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/loose/input.js
@@ -0,0 +1,9 @@
+function dec() {}
+
+class A {
+  @dec a;
+
+  @dec b = 123;
+
+  c = 456;
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/loose/options.json b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/loose/options.json
new file mode 100644
index 0000000000..8c83198953
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/loose/options.json
@@ -0,0 +1,7 @@
+{
+  "plugins": [
+    ["proposal-decorators", { "legacy": true }],
+    ["proposal-class-properties", { "loose": true }],
+    "transform-classes"
+  ]
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/loose/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/loose/output.js
new file mode 100644
index 0000000000..0a25e2b102
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/loose/output.js
@@ -0,0 +1,35 @@
+var _class, _descriptor, _descriptor2, _temp;
+
+function _initializerDefineProperty(target, property, descriptor, context) { if (!descriptor) return; Object.defineProperty(target, property, { enumerable: descriptor.enumerable, configurable: descriptor.configurable, writable: descriptor.writable, value: descriptor.initializer ? descriptor.initializer.call(context) : void 0 }); }
+
+function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
+
+function _applyDecoratedDescriptor(target, property, decorators, descriptor, context) { var desc = {}; Object.keys(descriptor).forEach(function (key) { desc[key] = descriptor[key]; }); desc.enumerable = !!desc.enumerable; desc.configurable = !!desc.configurable; if ('value' in desc || desc.initializer) { desc.writable = true; } desc = decorators.slice().reverse().reduce(function (desc, decorator) { return decorator(target, property, desc) || desc; }, desc); if (context && desc.initializer !== void 0) { desc.value = desc.initializer ? desc.initializer.call(context) : void 0; desc.initializer = undefined; } if (desc.initializer === void 0) { Object.defineProperty(target, property, desc); desc = null; } return desc; }
+
+function _initializerWarningHelper(descriptor, context) { throw new Error('Decorating class property failed. Please ensure that ' + 'proposal-class-properties is enabled and runs after the decorators transform.'); }
+
+function dec() {}
+
+let A = (_class = (_temp = function A() {
+  "use strict";
+
+  _classCallCheck(this, A);
+
+  _initializerDefineProperty(this, "a", _descriptor, this);
+
+  _initializerDefineProperty(this, "b", _descriptor2, this);
+
+  this.c = 456;
+}, _temp), (_descriptor = _applyDecoratedDescriptor(_class.prototype, "a", [dec], {
+  configurable: true,
+  enumerable: true,
+  writable: true,
+  initializer: null
+}), _descriptor2 = _applyDecoratedDescriptor(_class.prototype, "b", [dec], {
+  configurable: true,
+  enumerable: true,
+  writable: true,
+  initializer: function () {
+    return 123;
+  }
+})), _class);
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/strict/input.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/strict/input.js
new file mode 100644
index 0000000000..92298f2eb4
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/strict/input.js
@@ -0,0 +1,9 @@
+function dec() {}
+
+class A {
+  @dec a;
+
+  @dec b = 123;
+
+  c = 456;
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/strict/options.json b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/strict/options.json
new file mode 100644
index 0000000000..6f03cd7aad
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/strict/options.json
@@ -0,0 +1,7 @@
+{
+  "plugins": [
+    ["proposal-decorators", { "legacy": true }],
+    ["proposal-class-properties"],
+    "transform-classes"
+  ]
+}
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/strict/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/strict/output.js
new file mode 100644
index 0000000000..11cce4ac92
--- /dev/null
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/decorators-legacy-interop/strict/output.js
@@ -0,0 +1,37 @@
+var _class, _descriptor, _descriptor2, _temp;
+
+function _initializerDefineProperty(target, property, descriptor, context) { if (!descriptor) return; Object.defineProperty(target, property, { enumerable: descriptor.enumerable, configurable: descriptor.configurable, writable: descriptor.writable, value: descriptor.initializer ? descriptor.initializer.call(context) : void 0 }); }
+
+function _classCallCheck(instance, Constructor) { if (!(instance instanceof Constructor)) { throw new TypeError("Cannot call a class as a function"); } }
+
+function _defineProperty(obj, key, value) { if (key in obj) { Object.defineProperty(obj, key, { value: value, enumerable: true, configurable: true, writable: true }); } else { obj[key] = value; } return obj; }
+
+function _applyDecoratedDescriptor(target, property, decorators, descriptor, context) { var desc = {}; Object.keys(descriptor).forEach(function (key) { desc[key] = descriptor[key]; }); desc.enumerable = !!desc.enumerable; desc.configurable = !!desc.configurable; if ('value' in desc || desc.initializer) { desc.writable = true; } desc = decorators.slice().reverse().reduce(function (desc, decorator) { return decorator(target, property, desc) || desc; }, desc); if (context && desc.initializer !== void 0) { desc.value = desc.initializer ? desc.initializer.call(context) : void 0; desc.initializer = undefined; } if (desc.initializer === void 0) { Object.defineProperty(target, property, desc); desc = null; } return desc; }
+
+function _initializerWarningHelper(descriptor, context) { throw new Error('Decorating class property failed. Please ensure that ' + 'proposal-class-properties is enabled and runs after the decorators transform.'); }
+
+function dec() {}
+
+let A = (_class = (_temp = function A() {
+  "use strict";
+
+  _classCallCheck(this, A);
+
+  _initializerDefineProperty(this, "a", _descriptor, this);
+
+  _initializerDefineProperty(this, "b", _descriptor2, this);
+
+  _defineProperty(this, "c", 456);
+}, _temp), (_descriptor = _applyDecoratedDescriptor(_class.prototype, "a", [dec], {
+  configurable: true,
+  enumerable: true,
+  writable: true,
+  initializer: null
+}), _descriptor2 = _applyDecoratedDescriptor(_class.prototype, "b", [dec], {
+  configurable: true,
+  enumerable: true,
+  writable: true,
+  initializer: function () {
+    return 123;
+  }
+})), _class);
diff --git a/packages/babel-plugin-proposal-class-properties/test/fixtures/static-property-tdz/decorator-interop/output.js b/packages/babel-plugin-proposal-class-properties/test/fixtures/static-property-tdz/decorator-interop/output.js
index 2cd978b722..9a71c17528 100644
--- a/packages/babel-plugin-proposal-class-properties/test/fixtures/static-property-tdz/decorator-interop/output.js
+++ b/packages/babel-plugin-proposal-class-properties/test/fixtures/static-property-tdz/decorator-interop/output.js
@@ -12,7 +12,7 @@ function _createClass(Constructor, protoProps, staticProps) { if (protoProps) _d
 
 function _applyDecoratedDescriptor(target, property, decorators, descriptor, context) { var desc = {}; Object.keys(descriptor).forEach(function (key) { desc[key] = descriptor[key]; }); desc.enumerable = !!desc.enumerable; desc.configurable = !!desc.configurable; if ('value' in desc || desc.initializer) { desc.writable = true; } desc = decorators.slice().reverse().reduce(function (desc, decorator) { return decorator(target, property, desc) || desc; }, desc); if (context && desc.initializer !== void 0) { desc.value = desc.initializer ? desc.initializer.call(context) : void 0; desc.initializer = undefined; } if (desc.initializer === void 0) { Object.defineProperty(target, property, desc); desc = null; } return desc; }
 
-function _initializerWarningHelper(descriptor, context) { throw new Error('Decorating class property failed. Please ensure that ' + 'proposal-class-properties is enabled and set to use loose mode. ' + 'To use proposal-class-properties in spec mode with decorators, wait for ' + 'the next major version of decorators in stage 2.'); }
+function _initializerWarningHelper(descriptor, context) { throw new Error('Decorating class property failed. Please ensure that ' + 'proposal-class-properties is enabled and runs after the decorators transform.'); }
 
 function dec() {}
 
diff --git a/packages/babel-plugin-proposal-decorators/src/transformer-legacy.js b/packages/babel-plugin-proposal-decorators/src/transformer-legacy.js
index d6160db14c..ca23bad248 100644
--- a/packages/babel-plugin-proposal-decorators/src/transformer-legacy.js
+++ b/packages/babel-plugin-proposal-decorators/src/transformer-legacy.js
@@ -290,4 +290,24 @@ export default {
       ]),
     );
   },
+
+  CallExpression(path, state) {
+    if (path.node.arguments.length !== 3) return;
+    if (!WARNING_CALLS.has(path.node.arguments[2])) return;
+
+    // If the class properties plugin isn't enabled, this line will add an unused helper
+    // to the code. It's not ideal, but it's ok since the configuration is not valid anyway.
+    if (path.node.callee.name !== state.addHelper("defineProperty").name) {
+      return;
+    }
+
+    path.replaceWith(
+      t.callExpression(state.addHelper("initializerDefineProperty"), [
+        t.cloneNode(path.get("arguments")[0].node),
+        t.cloneNode(path.get("arguments")[1].node),
+        t.cloneNode(path.get("arguments.2.arguments")[0].node),
+        t.cloneNode(path.get("arguments.2.arguments")[1].node),
+      ]),
+    );
+  },
 };
diff --git a/packages/babel-plugin-transform-typescript/test/fixtures/class/abstract-class-decorated-parameter/output.mjs b/packages/babel-plugin-transform-typescript/test/fixtures/class/abstract-class-decorated-parameter/output.mjs
index e8982550ef..58aa80d516 100644
--- a/packages/babel-plugin-transform-typescript/test/fixtures/class/abstract-class-decorated-parameter/output.mjs
+++ b/packages/babel-plugin-transform-typescript/test/fixtures/class/abstract-class-decorated-parameter/output.mjs
@@ -2,7 +2,7 @@ var _class, _descriptor;
 
 function _applyDecoratedDescriptor(target, property, decorators, descriptor, context) { var desc = {}; Object.keys(descriptor).forEach(function (key) { desc[key] = descriptor[key]; }); desc.enumerable = !!desc.enumerable; desc.configurable = !!desc.configurable; if ('value' in desc || desc.initializer) { desc.writable = true; } desc = decorators.slice().reverse().reduce(function (desc, decorator) { return decorator(target, property, desc) || desc; }, desc); if (context && desc.initializer !== void 0) { desc.value = desc.initializer ? desc.initializer.call(context) : void 0; desc.initializer = undefined; } if (desc.initializer === void 0) { Object.defineProperty(target, property, desc); desc = null; } return desc; }
 
-function _initializerWarningHelper(descriptor, context) { throw new Error('Decorating class property failed. Please ensure that ' + 'proposal-class-properties is enabled and set to use loose mode. ' + 'To use proposal-class-properties in spec mode with decorators, wait for ' + 'the next major version of decorators in stage 2.'); }
+function _initializerWarningHelper(descriptor, context) { throw new Error('Decorating class property failed. Please ensure that ' + 'proposal-class-properties is enabled and runs after the decorators transform.'); }
 
 import { observable } from 'mobx';
 let Foo = (_class = class Foo {