diff --git a/packages/eslint-plugin/docs/rules/no-namespace.md b/packages/eslint-plugin/docs/rules/no-namespace.md
index d82ad724b7b..b155c2aeb30 100644
--- a/packages/eslint-plugin/docs/rules/no-namespace.md
+++ b/packages/eslint-plugin/docs/rules/no-namespace.md
@@ -51,6 +51,14 @@ Examples of **correct** code for the `{ "allowDeclarations": true }` option:
 declare module 'foo' {}
 declare module foo {}
 declare namespace foo {}
+
+declare global {
+  namespace foo {}
+}
+
+declare module foo {
+  namespace foo {}
+}
 ```
 
 Examples of **incorrect** code for the `{ "allowDeclarations": false }` option:
diff --git a/packages/eslint-plugin/src/rules/no-namespace.ts b/packages/eslint-plugin/src/rules/no-namespace.ts
index 3698b4f941b..f8cc05b1047 100644
--- a/packages/eslint-plugin/src/rules/no-namespace.ts
+++ b/packages/eslint-plugin/src/rules/no-namespace.ts
@@ -50,6 +50,14 @@ export default util.createRule<Options, MessageIds>({
   create(context, [{ allowDeclarations, allowDefinitionFiles }]) {
     const filename = context.getFilename();
 
+    function isDeclaration(node: TSESTree.TSModuleDeclaration): boolean {
+      return (
+        node.declare === true ||
+        (node.parent!.parent?.type === AST_NODE_TYPES.TSModuleDeclaration &&
+          isDeclaration(node.parent!.parent))
+      );
+    }
+
     return {
       "TSModuleDeclaration[global!=true][id.type='Identifier']"(
         node: TSESTree.TSModuleDeclaration,
@@ -58,7 +66,7 @@ export default util.createRule<Options, MessageIds>({
           (node.parent &&
             node.parent.type === AST_NODE_TYPES.TSModuleDeclaration) ||
           (allowDefinitionFiles && util.isDefinitionFile(filename)) ||
-          (allowDeclarations && node.declare === true)
+          (allowDeclarations && isDeclaration(node))
         ) {
           return;
         }
diff --git a/packages/eslint-plugin/tests/rules/no-namespace.test.ts b/packages/eslint-plugin/tests/rules/no-namespace.test.ts
index e175f5895df..3bb9079b3fc 100644
--- a/packages/eslint-plugin/tests/rules/no-namespace.test.ts
+++ b/packages/eslint-plugin/tests/rules/no-namespace.test.ts
@@ -17,6 +17,42 @@ ruleTester.run('no-namespace', rule, {
       code: 'declare namespace foo {}',
       options: [{ allowDeclarations: true }],
     },
+    {
+      code: `
+declare global {
+  namespace foo {}
+}
+      `,
+      options: [{ allowDeclarations: true }],
+    },
+    {
+      code: `
+declare module foo {
+  namespace bar {}
+}
+      `,
+      options: [{ allowDeclarations: true }],
+    },
+    {
+      code: `
+declare global {
+  namespace foo {
+    namespace bar {}
+  }
+}
+      `,
+      options: [{ allowDeclarations: true }],
+    },
+    {
+      code: `
+declare namespace foo {
+  namespace bar {
+    namespace baz {}
+  }
+}
+      `,
+      options: [{ allowDeclarations: true }],
+    },
     {
       filename: 'test.d.ts',
       code: 'namespace foo {}',
@@ -71,6 +107,28 @@ ruleTester.run('no-namespace', rule, {
         },
       ],
     },
+    {
+      code: 'module foo {}',
+      options: [{ allowDeclarations: true }],
+      errors: [
+        {
+          messageId: 'moduleSyntaxIsPreferred',
+          line: 1,
+          column: 1,
+        },
+      ],
+    },
+    {
+      code: 'namespace foo {}',
+      options: [{ allowDeclarations: true }],
+      errors: [
+        {
+          messageId: 'moduleSyntaxIsPreferred',
+          line: 1,
+          column: 1,
+        },
+      ],
+    },
     {
       code: 'declare module foo {}',
       errors: [