Yahoo and ABC News announced Monday that they will co-run news Web sites.

The partnership was announced on ABC’s “Good Morning America,” and is expected to reach 100 million Americans on PCs and mobile devices, the companies said.

The news partnership will produce content for the Web, and provide more Web-first programming and content. The two companies are calling the new relationship a “game-changer.”

Yahoo and ABC already had a video sharing agreement.

Under the new agreement, ABC will provide most of the content for Yahoo News and will also provide a new Web series featuring ABC news anchors and correspondents.

Yahoo News and ABC News teams will work together to develop content, though both will remain in control of their own editorial content.

The partnership kicks off Monday with the launch of a new Web site for “Good Morning America,” a co-branded, stand-alone site that the companies hope will encourage Web engagement.

Related stories:

More technology coverage from The Post

What’s next for Yahoo?

Carol Bartz lashes out at Yahoo board