diff --git a/app/controllers/users_controller.rb b/app/controllers/users_controller.rb
index 11bd28b7a..74deb686b 100644
--- a/app/controllers/users_controller.rb
+++ b/app/controllers/users_controller.rb
@@ -19,7 +19,10 @@ class UsersController < ApplicationController
         format.json { head :no_content }
       end
     else
-      render :edit, status: :unprocessable_entity
+      respond_to do |format|
+        format.html { render :edit, status: :unprocessable_entity }
+        format.json { render json: @user.errors, status: :unprocessable_entity }
+      end
     end
   end
 
diff --git a/docs/API.md b/docs/API.md
index 47473a845..107521596 100644
--- a/docs/API.md
+++ b/docs/API.md
@@ -83,6 +83,29 @@ else
 end
 ```
 
+## Error Responses
+
+When a request fails, the API response will communicate the source of the problem through the HTTP status code.
+
+| Status Code | Description |
+|-------------|-------------|
+| `400 Bad Request` | The request was malformed or missing required parameters |
+| `401 Unauthorized` | Authentication failed or access token is invalid |
+| `403 Forbidden` | You don't have permission to perform this action |
+| `404 Not Found` | The requested resource doesn't exist or you don't have access to it |
+| `422 Unprocessable Entity` | Validation failed (see error response format above) |
+| `500 Internal Server Error` | An unexpected error occurred on the server |
+
+If a request contains invalid data for fields, such as entering a string into a number field, in most cases the API will respond with a `500 Internal Server Error`. Clients are expected to perform some validation on their end before making a request.
+
+Validation error will produce a `422 Unprocessable Entity` which will sometimes be accompanied by details about the validation errors:
+
+```json
+{
+  "avatar": ["must be a JPEG, PNG, GIF, or WebP image"]
+}
+```
+
 ## Pagination
 
 All endpoints that return a list of items are paginated. The page size can vary from endpoint to endpoint,
diff --git a/test/controllers/users_controller_test.rb b/test/controllers/users_controller_test.rb
index bf6854a0d..20008dcee 100644
--- a/test/controllers/users_controller_test.rb
+++ b/test/controllers/users_controller_test.rb
@@ -112,6 +112,17 @@ class UsersControllerTest < ActionDispatch::IntegrationTest
     assert_equal "New David", users(:david).reload.name
   end
 
+  test "update as JSON with invalid avatar returns errors" do
+    sign_in_as :kevin
+
+    svg_file = fixture_file_upload("avatar.svg", "image/svg+xml")
+
+    put user_path(users(:kevin), format: :json), params: { user: { avatar: svg_file } }
+
+    assert_response :unprocessable_entity
+    assert @response.parsed_body["avatar"].present?
+  end
+
   test "destroy as JSON" do
     sign_in_as :kevin