diff --git a/src/validator/validator.test.ts b/src/validator/validator.test.ts
index cc3b6d957..aad7785bb 100644
--- a/src/validator/validator.test.ts
+++ b/src/validator/validator.test.ts
@@ -260,6 +260,21 @@ describe('FormData', () => {
       'foo[]': ['bar1', 'bar2'],
     })
   })
+
+  it('Should return `foo` as an array if multiple values are appended', async () => {
+    const form = new FormData()
+    form.append('foo', 'bar1')
+    form.append('foo', 'bar2')
+    form.append('foo', 'bar3')
+    const res = await app.request('/post', {
+      method: 'POST',
+      body: form,
+    })
+    expect(res.status).toBe(200)
+    expect(await res.json()).toEqual({
+      foo: ['bar1', 'bar2', 'bar3'],
+    })
+  })
 })
 
 describe('Malformed FormData request', () => {
diff --git a/src/validator/validator.ts b/src/validator/validator.ts
index b9c346a35..bea6dafec 100644
--- a/src/validator/validator.ts
+++ b/src/validator/validator.ts
@@ -108,11 +108,11 @@ export const validator = <
         const form: BodyData<{ all: true }> = {}
         formData.forEach((value, key) => {
           if (key.endsWith('[]')) {
-            if (form[key] === undefined) {
-              form[key] = [value]
-            } else if (Array.isArray(form[key])) {
-              ;(form[key] as unknown[]).push(value)
-            }
+            ;((form[key] ??= []) as unknown[]).push(value)
+          } else if (Array.isArray(form[key])) {
+            ;(form[key] as unknown[]).push(value)
+          } else if (key in form) {
+            form[key] = [form[key] as string | File, value]
           } else {
             form[key] = value
           }