diff --git a/packages/gatsby/src/bootstrap/load-plugins/__tests__/load-plugins.ts b/packages/gatsby/src/bootstrap/load-plugins/__tests__/load-plugins.ts
index 7cae03721ecc6..6902016fceefa 100644
--- a/packages/gatsby/src/bootstrap/load-plugins/__tests__/load-plugins.ts
+++ b/packages/gatsby/src/bootstrap/load-plugins/__tests__/load-plugins.ts
@@ -576,6 +576,58 @@ describe(`Load plugins`, () => {
       expect(mockProcessExit).toHaveBeenCalledWith(1)
     })
 
+    it(`validates subplugin schemas (if not in options.plugins)`, async () => {
+      await loadPlugins(
+        {
+          plugins: [
+            {
+              resolve: `gatsby-plugin-mdx`,
+              options: {
+                gatsbyRemarkPlugins: [
+                  {
+                    resolve: `gatsby-remark-autolink-headers`,
+                    options: {
+                      maintainCase: `should be boolean`,
+                    },
+                  },
+                ],
+              },
+            },
+          ],
+        },
+        process.cwd()
+      )
+
+      expect(reporter.error as jest.Mock).toHaveBeenCalledTimes(1)
+      expect((reporter.error as jest.Mock).mock.calls[0])
+        .toMatchInlineSnapshot(`
+        Array [
+          Object {
+            "context": Object {
+              "configDir": null,
+              "pluginName": "gatsby-remark-autolink-headers",
+              "validationErrors": Array [
+                Object {
+                  "context": Object {
+                    "key": "maintainCase",
+                    "label": "maintainCase",
+                    "value": "should be boolean",
+                  },
+                  "message": "\\"maintainCase\\" must be a boolean",
+                  "path": Array [
+                    "maintainCase",
+                  ],
+                  "type": "boolean.base",
+                },
+              ],
+            },
+            "id": "11331",
+          },
+        ]
+      `)
+      expect(mockProcessExit).toHaveBeenCalledWith(1)
+    })
+
     it(`subplugins are resolved using "main" in package.json`, async () => {
       // in fixtures/subplugins/node_modules/gatsby-plugin-child-with-main/package.json
       // "main" field points to "lib/index.js"
diff --git a/packages/gatsby/src/bootstrap/load-plugins/validate.ts b/packages/gatsby/src/bootstrap/load-plugins/validate.ts
index 5e091913c3cf2..6bca15195da53 100644
--- a/packages/gatsby/src/bootstrap/load-plugins/validate.ts
+++ b/packages/gatsby/src/bootstrap/load-plugins/validate.ts
@@ -181,26 +181,25 @@ export async function handleBadExports({
   }
 }
 
-interface ISubPluginCustomReturn {
-  resolve: string
-  modulePath: string
-  options: {
-    [key: string]: unknown
-  }
-  module: any
-}
+const addModuleImportAndValidateOptions =
+  (rootDir: string, incErrors: (inc: number) => void) =>
+  async (value: Array<IPluginRefObject>): Promise<Array<IPluginRefObject>> => {
+    for (const plugin of value) {
+      if (plugin.modulePath) {
+        const importedModule = await import(
+          maybeAddFileProtocol(plugin.modulePath)
+        )
+        const pluginModule = preferDefault(importedModule)
+        plugin.module = pluginModule
+      }
+    }
 
-const addModuleImport = async (
-  value: Array<ISubPluginCustomReturn>
-): Promise<Array<ISubPluginCustomReturn>> => {
-  for (const plugin of value) {
-    const importedModule = await import(maybeAddFileProtocol(plugin.modulePath))
-    const pluginModule = preferDefault(importedModule)
-    plugin.module = pluginModule
-  }
+    const { errors: subErrors, plugins: subPlugins } =
+      await validatePluginsOptions(value as Array<IPluginRefObject>, rootDir)
 
-  return value
-}
+    incErrors(subErrors)
+    return subPlugins
+  }
 
 async function validatePluginsOptions(
   plugins: Array<IPluginRefObject>,
@@ -290,7 +289,15 @@ async function validatePluginsOptions(
                 })
               }, `Gatsby specific subplugin validation`)
               .default([])
-              .external(addModuleImport, `add module key to subplugin`),
+              .external(
+                addModuleImportAndValidateOptions(
+                  rootDir,
+                  (inc: number): void => {
+                    errors += inc
+                  }
+                ),
+                `add module key to subplugin`
+              ),
             args: (schema: any, args: any): any => {
               if (
                 args?.entry &&
@@ -364,8 +371,8 @@ async function validatePluginsOptions(
           // We do not increment errors++ here as we do not want to process.exit if there are only warnings
         }
 
-        // Validate subplugins
-        if (plugin.options?.plugins) {
+        // Validate subplugins if they weren't handled already
+        if (!subPluginPaths.has(`plugins`) && plugin.options?.plugins) {
           const { errors: subErrors, plugins: subPlugins } =
             await validatePluginsOptions(
               plugin.options.plugins as Array<IPluginRefObject>,