diff --git a/CHANGELOG.md b/CHANGELOG.md
index e8493698859b..2fc5a7d18573 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -10,6 +10,7 @@
 - `[jest-runtime]` [**BREAKING**] Do not inject `global` variable into module wrapper ([#10644](https://github.com/facebook/jest/pull/10644))
 - `[jest-transform]` Show enhanced `SyntaxError` message for all `SyntaxError`s ([#10749](https://github.com/facebook/jest/pull/10749))
 - `[jest-transform]` [**BREAKING**] Refactor API to pass an options bag around rather than multiple boolean options ([#10753](https://github.com/facebook/jest/pull/10753))
+- `[jest-transform]` [**BREAKING**] All transformers must now return `{code: string, map?: SourceMap | string | null}`. Returning a string is no longer supported ([#10773](https://github.com/facebook/jest/pull/10773))
 
 ### Chore & Maintenance
 
diff --git a/packages/babel-jest/src/index.ts b/packages/babel-jest/src/index.ts
index 07181095ccc7..91c55dd0965b 100644
--- a/packages/babel-jest/src/index.ts
+++ b/packages/babel-jest/src/index.ts
@@ -164,7 +164,7 @@ const createTransformer = (
         }
       }
 
-      return src;
+      return {code: src};
     },
   };
 };
diff --git a/packages/jest-repl/src/cli/repl.ts b/packages/jest-repl/src/cli/repl.ts
index 831b2323489a..280ae085159a 100644
--- a/packages/jest-repl/src/cli/repl.ts
+++ b/packages/jest-repl/src/cli/repl.ts
@@ -30,10 +30,7 @@ const evalCommand: repl.REPLEval = (
         jestGlobalConfig.replname || 'jest.js',
         jestProjectConfig,
       );
-      cmd =
-        typeof transformResult === 'string'
-          ? transformResult
-          : transformResult.code;
+      cmd = transformResult.code;
     }
     result = runInThisContext(cmd);
   } catch (e) {
diff --git a/packages/jest-transform/src/ScriptTransformer.ts b/packages/jest-transform/src/ScriptTransformer.ts
index f99310ab3911..2161aa1dec90 100644
--- a/packages/jest-transform/src/ScriptTransformer.ts
+++ b/packages/jest-transform/src/ScriptTransformer.ts
@@ -208,10 +208,7 @@ export default class ScriptTransformer {
     canMapToInput: boolean,
     options: TransformOptions,
   ): TransformedSource {
-    const inputCode = typeof input === 'string' ? input : input.code;
-    const inputMap = typeof input === 'string' ? null : input.map;
-
-    const result = babelTransform(inputCode, {
+    const result = babelTransform(input.code, {
       auxiliaryCommentBefore: ' istanbul ignore next ',
       babelrc: false,
       caller: {
@@ -232,7 +229,7 @@ export default class ScriptTransformer {
             cwd: this._config.rootDir,
             exclude: [],
             extension: false,
-            inputSourceMap: inputMap,
+            inputSourceMap: input.map,
             useInlineSourceMaps: false,
           },
         ],
@@ -240,7 +237,7 @@ export default class ScriptTransformer {
       sourceMaps: canMapToInput ? 'both' : false,
     });
 
-    if (result && result.code) {
+    if (result?.code) {
       return result as TransformResult;
     }
 
@@ -297,14 +294,13 @@ export default class ScriptTransformer {
         options,
       );
 
-      if (typeof processed === 'string') {
-        transformed.code = processed;
-      } else if (processed != null && typeof processed.code === 'string') {
+      if (processed != null && typeof processed.code === 'string') {
         transformed = processed;
       } else {
         throw new TypeError(
-          "Jest: a transform's `process` function must return a string, " +
-            'or an object with `code` key containing this string.',
+          "Jest: a transform's `process` function must return an object with " +
+            '`code` key containing a string and an optional `map` key with ' +
+            'the source map.',
         );
       }
     }
@@ -349,9 +345,8 @@ export default class ScriptTransformer {
         options,
       );
 
-      code =
-        typeof instrumented === 'string' ? instrumented : instrumented.code;
-      map = typeof instrumented === 'string' ? null : instrumented.map;
+      code = instrumented.code;
+      map = instrumented.map;
     } else {
       code = transformed.code;
     }
diff --git a/packages/jest-transform/src/types.ts b/packages/jest-transform/src/types.ts
index 979c16c8ad42..521e66ef0bca 100644
--- a/packages/jest-transform/src/types.ts
+++ b/packages/jest-transform/src/types.ts
@@ -31,10 +31,10 @@ interface FixedRawSourceMap extends Omit<RawSourceMap, 'version'> {
   version: number;
 }
 
-// TODO: For Jest 26 normalize this (always structured data, never a string)
-export type TransformedSource =
-  | {code: string; map?: FixedRawSourceMap | string | null}
-  | string;
+export type TransformedSource = {
+  code: string;
+  map?: FixedRawSourceMap | string | null;
+};
 
 export type TransformResult = TransformTypes.TransformResult;