YouTube said it is planning changes to give users more context for videos promoting conspiracy theories or state-sponsored content, the latest effort by an internet giant to clean up its platform amid criticism over its role in spreading misinformation.

YouTube said starting Friday it would label all videos coming from what it identifies as state-funded broadcasters, a category that would include even the U.S.’s Public Broadcasting Service, or PBS. The step is significant in part because YouTube has been a major conduit for...