The firms will also create what they are calling the OpenSocial Foundation, which is planned to be an independent non-profit body that will provide technology guidelines, set new industry specifications for social web application development and ensure that all members share influence over its future direction.
With Yahoo! joining the pressure will be on Facebook, which has yet to join and has continued to develop its own social media standards. Yahoo! bidder Microsoft is also not a supporter of OpenSocial.
The News Corporation owned MySpace and Google's own less well known social media network, Orkut, recently launched developer platforms that are compatible with OpenSocial.
Yahoo! said it was supporting community-driven industry specifications and expected that OpenSocial will fuel innovation and make the web more relevant and more enjoyable to millions of users.
Wade Chambers, vice president, platforms at Yahoo!, said: "Our support will expand the opportunity for developers and publishers to benefit from an open and increasingly social web."
Yahoo! is seen as an important addition to the OpenSocial movement and underscores its status as a community-driven specification.
Joe Kraus, director of product management, Google, said: "The formation of this foundation will ensure that it remains so in perpetuity. Developers and websites should feel secure that OpenSocial will be forever free and open."
The OpenSocial Foundation website will serve as the portal for the community to find all information about OpenSocial and the foundation as they evolve. Developers and site owners can visit the site for the latest specifications, links to other resources, and the opportunity to get involved.
Other members of the OpenSocial community include Engage.com, Friendster, LinkedIn, Oracle, Plaxo and Salesforce.com.