diff --git a/packages/babel-core/src/config/validation/options.js b/packages/babel-core/src/config/validation/options.js
index 507b183fa07a..e2c3bfd2dafb 100644
--- a/packages/babel-core/src/config/validation/options.js
+++ b/packages/babel-core/src/config/validation/options.js
@@ -334,6 +334,7 @@ export type NestingPath = RootPath | OverridesPath | EnvPath;
 export const assumptionsNames = new Set<string>([
   "mutableTemplateObject",
   "newableArrowFunctions",
+  "setClassMethods",
   "setPublicClassFields",
 ]);
 
diff --git a/packages/babel-plugin-transform-classes/src/index.js b/packages/babel-plugin-transform-classes/src/index.js
index 8f0ecfc202ca..5fbd90f82a32 100644
--- a/packages/babel-plugin-transform-classes/src/index.js
+++ b/packages/babel-plugin-transform-classes/src/index.js
@@ -21,6 +21,8 @@ export default declare((api, options) => {
 
   const { loose } = options;
 
+  const setClassMethods = options.loose || api.assumption("setClassMethods");
+
   // todo: investigate traversal requeueing
   const VISITED = Symbol();
 
@@ -58,7 +60,9 @@ export default declare((api, options) => {
         node[VISITED] = true;
 
         path.replaceWith(
-          transformClass(path, state.file, builtinClasses, loose),
+          transformClass(path, state.file, builtinClasses, loose, {
+            setClassMethods,
+          }),
         );
 
         if (path.isCallExpression()) {
diff --git a/packages/babel-plugin-transform-classes/src/transformClass.js b/packages/babel-plugin-transform-classes/src/transformClass.js
index 5b8f5f5171b1..00096aeac65e 100644
--- a/packages/babel-plugin-transform-classes/src/transformClass.js
+++ b/packages/babel-plugin-transform-classes/src/transformClass.js
@@ -12,6 +12,8 @@ import addCreateSuperHelper from "./inline-createSuper-helpers";
 
 type ReadonlySet<T> = Set<T> | { has(val: T): boolean };
 
+type ClassAssumptions = { setClassMethods: boolean };
+
 function buildConstructor(classRef, constructorBody, node) {
   const func = t.functionDeclaration(
     t.cloneNode(classRef),
@@ -27,6 +29,7 @@ export default function transformClass(
   file: any,
   builtinClasses: ReadonlySet<string>,
   isLoose: boolean,
+  assumptions: ClassAssumptions,
 ) {
   const classState = {
     parent: undefined,
@@ -431,7 +434,7 @@ export default function transformClass(
   }
 
   function processMethod(node, scope) {
-    if (classState.isLoose && !node.decorators) {
+    if (assumptions.setClassMethods && !node.decorators) {
       // use assignments instead of define properties for loose classes
       let { classRef } = classState;
       if (!node.static) {
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/literal-key/input.js b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/literal-key/input.js
new file mode 100644
index 000000000000..e3ad20145b86
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/literal-key/input.js
@@ -0,0 +1,5 @@
+class Foo {
+  "bar"() {
+
+  }
+}
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/literal-key/output.js b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/literal-key/output.js
new file mode 100644
index 000000000000..610fbb5a157e
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/literal-key/output.js
@@ -0,0 +1,13 @@
+var Foo = /*#__PURE__*/function () {
+  "use strict";
+
+  function Foo() {
+    babelHelpers.classCallCheck(this, Foo);
+  }
+
+  var _proto = Foo.prototype;
+
+  _proto["bar"] = function bar() {};
+
+  return Foo;
+}();
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-return-type-annotation/input.js b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-return-type-annotation/input.js
new file mode 100644
index 000000000000..918e7469b6c7
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-return-type-annotation/input.js
@@ -0,0 +1,6 @@
+// @flow
+class C {
+  m(x: number): string {
+    return 'a';
+  }
+}
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-return-type-annotation/options.json b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-return-type-annotation/options.json
new file mode 100644
index 000000000000..1f9617ec390b
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-return-type-annotation/options.json
@@ -0,0 +1,10 @@
+{
+  "plugins": [
+    "external-helpers",
+    "transform-function-name",
+    ["transform-classes", { "loose": true }],
+    "transform-spread",
+    "transform-block-scoping",
+    "syntax-flow"
+  ]
+}
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-return-type-annotation/output.js b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-return-type-annotation/output.js
new file mode 100644
index 000000000000..45403829143d
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-return-type-annotation/output.js
@@ -0,0 +1,14 @@
+// @flow
+var C = /*#__PURE__*/function () {
+  "use strict";
+
+  function C() {}
+
+  var _proto = C.prototype;
+
+  _proto.m = function m(x: number): string {
+    return 'a';
+  };
+
+  return C;
+}();
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-reuse-prototype/input.js b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-reuse-prototype/input.js
new file mode 100644
index 000000000000..1c08a3644980
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-reuse-prototype/input.js
@@ -0,0 +1,5 @@
+class Test {
+  a() {}
+  static b() {}
+  c() {}
+}
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-reuse-prototype/output.js b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-reuse-prototype/output.js
new file mode 100644
index 000000000000..844b0d7655e7
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/method-reuse-prototype/output.js
@@ -0,0 +1,17 @@
+var Test = /*#__PURE__*/function () {
+  "use strict";
+
+  function Test() {
+    babelHelpers.classCallCheck(this, Test);
+  }
+
+  var _proto = Test.prototype;
+
+  _proto.a = function a() {};
+
+  Test.b = function b() {};
+
+  _proto.c = function c() {};
+
+  return Test;
+}();
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/options.json b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/options.json
new file mode 100644
index 000000000000..62562eb645d7
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/options.json
@@ -0,0 +1,12 @@
+{
+  "plugins": [
+    ["external-helpers", { "helperVersion": "7.100.0" }],
+    "transform-function-name",
+    "transform-classes",
+    ["transform-spread", { "loose": true }],
+    "transform-block-scoping"
+  ],
+  "assumptions": {
+    "setClassMethods": true
+  }
+}
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/program-with-use-strict-directive/input.js b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/program-with-use-strict-directive/input.js
new file mode 100644
index 000000000000..4bbf0638b3be
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/program-with-use-strict-directive/input.js
@@ -0,0 +1,7 @@
+"use strict";
+
+class Foo {
+    method() {
+        
+    }
+}
\ No newline at end of file
diff --git a/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/program-with-use-strict-directive/output.js b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/program-with-use-strict-directive/output.js
new file mode 100644
index 000000000000..96acb356c59a
--- /dev/null
+++ b/packages/babel-plugin-transform-classes/test/fixtures/assumption-setClassMethods/program-with-use-strict-directive/output.js
@@ -0,0 +1,13 @@
+"use strict";
+
+var Foo = /*#__PURE__*/function () {
+  function Foo() {
+    babelHelpers.classCallCheck(this, Foo);
+  }
+
+  var _proto = Foo.prototype;
+
+  _proto.method = function method() {};
+
+  return Foo;
+}();