Alphabet Inc.’s Google plans to prohibit fake-news websites from using its ad-selling software, a move that could crimp revenue at those sites.

Google said Monday that it is updating its policies to ban Google ads being placed “on pages that misrepresent, misstate, or conceal information about the publisher, the publisher’s content, or the primary purpose” of the website. The policy would include sites that distribute false news, a Google spokeswoman said.

False...