diff --git a/packages/babel-plugin-transform-typescript/src/namespace.ts b/packages/babel-plugin-transform-typescript/src/namespace.ts
index 2a24f156deac..186da7c3c77c 100644
--- a/packages/babel-plugin-transform-typescript/src/namespace.ts
+++ b/packages/babel-plugin-transform-typescript/src/namespace.ts
@@ -1,4 +1,5 @@
 import { template, types as t } from "@babel/core";
+import type { NodePath } from "@babel/traverse";
 
 export default function transpileNamespace(path, t, allowNamespaces) {
   if (path.node.declare || path.node.id.type === "StringLiteral") {
@@ -102,11 +103,34 @@ function handleVariableDeclaration(
   return [node, t.expressionStatement(t.sequenceExpression(assignments))];
 }
 
-function handleNested(path, t, node, parentExport?) {
+function buildNestedAmbiendModuleError(path: NodePath, node: t.Node) {
+  throw path.hub.buildError(
+    node,
+    "Ambient modules cannot be nested in other modules or namespaces.",
+    Error,
+  );
+}
+
+function handleNested(
+  path: NodePath,
+  t: typeof import("@babel/types"),
+  node: t.TSModuleDeclaration,
+  parentExport?,
+) {
   const names = new Set();
   const realName = node.id;
+  t.assertIdentifier(realName);
+
   const name = path.scope.generateUid(realName.name);
-  const namespaceTopLevel = node.body.body;
+
+  const namespaceTopLevel: t.Statement[] = t.isTSModuleBlock(node.body)
+    ? node.body.body
+    : // We handle `namespace X.Y {}` as if it was
+      //   namespace X {
+      //     export namespace Y {}
+      //   }
+      [t.exportNamedDeclaration(node.body)];
+
   for (let i = 0; i < namespaceTopLevel.length; i++) {
     const subNode = namespaceTopLevel[i];
 
@@ -114,6 +138,10 @@ function handleNested(path, t, node, parentExport?) {
     // declarations require further transformation.
     switch (subNode.type) {
       case "TSModuleDeclaration": {
+        if (!t.isIdentifier(subNode.id)) {
+          throw buildNestedAmbiendModuleError(path, subNode);
+        }
+
         const transformed = handleNested(path, t, subNode);
         const moduleName = subNode.id.name;
         if (names.has(moduleName)) {
@@ -181,6 +209,10 @@ function handleNested(path, t, node, parentExport?) {
         break;
       }
       case "TSModuleDeclaration": {
+        if (!t.isIdentifier(subNode.declaration.id)) {
+          throw buildNestedAmbiendModuleError(path, subNode.declaration);
+        }
+
         const transformed = handleNested(
           path,
           t,
@@ -204,7 +236,7 @@ function handleNested(path, t, node, parentExport?) {
   }
 
   // {}
-  let fallthroughValue = t.objectExpression([]);
+  let fallthroughValue: t.Expression = t.objectExpression([]);
 
   if (parentExport) {
     const memberExpr = t.memberExpression(parentExport, realName);
diff --git a/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested-exported/input.ts b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested-exported/input.ts
new file mode 100644
index 000000000000..58db7e4f66d9
--- /dev/null
+++ b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested-exported/input.ts
@@ -0,0 +1,3 @@
+namespace X {
+  export module "m" {}
+}
diff --git a/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested-exported/options.json b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested-exported/options.json
new file mode 100644
index 000000000000..653571301113
--- /dev/null
+++ b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested-exported/options.json
@@ -0,0 +1,3 @@
+{
+  "throws": "Ambient modules cannot be nested in other modules or namespaces."
+}
diff --git a/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested/input.ts b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested/input.ts
new file mode 100644
index 000000000000..35d3d38cb1f6
--- /dev/null
+++ b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested/input.ts
@@ -0,0 +1,3 @@
+namespace X {
+  module "m" {}
+}
diff --git a/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested/options.json b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested/options.json
new file mode 100644
index 000000000000..653571301113
--- /dev/null
+++ b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/ambient-module-nested/options.json
@@ -0,0 +1,3 @@
+{
+  "throws": "Ambient modules cannot be nested in other modules or namespaces."
+}
diff --git a/packages/babel-plugin-transform-typescript/test/fixtures/namespace/nested-shorthand/input.ts b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/nested-shorthand/input.ts
new file mode 100644
index 000000000000..8a39ba3d5e72
--- /dev/null
+++ b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/nested-shorthand/input.ts
@@ -0,0 +1,7 @@
+namespace X.Y {
+  export const Z = 1;
+}
+
+namespace proj.data.util.api {
+  export const X = 1;
+}
diff --git a/packages/babel-plugin-transform-typescript/test/fixtures/namespace/nested-shorthand/output.mjs b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/nested-shorthand/output.mjs
new file mode 100644
index 000000000000..8c99a195f160
--- /dev/null
+++ b/packages/babel-plugin-transform-typescript/test/fixtures/namespace/nested-shorthand/output.mjs
@@ -0,0 +1,27 @@
+let X;
+
+(function (_X) {
+  let Y;
+
+  (function (_Y) {
+    const Z = _Y.Z = 1;
+  })(Y || (Y = _X.Y || (_X.Y = {})));
+})(X || (X = {}));
+
+let proj;
+
+(function (_proj) {
+  let data;
+
+  (function (_data) {
+    let util;
+
+    (function (_util) {
+      let api;
+
+      (function (_api) {
+        const X = _api.X = 1;
+      })(api || (api = _util.api || (_util.api = {})));
+    })(util || (util = _data.util || (_data.util = {})));
+  })(data || (data = _proj.data || (_proj.data = {})));
+})(proj || (proj = {}));