A group of San Francisco Bay Area philanthropists has pledged to spend half a billion dollars to protect and expand affordable housing in the region, according to an announcement on Thursday. A central player in the project is the philanthropy founded by Facebook’s chief executive, Mark Zuckerberg, and his wife, Dr. Priscilla Chan.

The effort, called the Partnership for the Bay’s Future, is meant to combat the dizzying ascent of home prices that has run parallel with Silicon Valley’s latest expansion. In November, the median sale price for a home in the Bay Area was $815,000, according to the research firm CoreLogic.

The money will seed an investment fund that will work to preserve housing for 175,000 families and add capacity for 8,000 new homes over the next decade through loans and other assistance to community groups. A separate $40 million fund will make grants to local governments and other groups trying to devise policies to protect affordable housing on a large scale.

In a news release announcing the partnership, California’s governor, Gavin Newsom, called the move a “bold action.” Mr. Newsom, who has criticized tech companies for not doing enough to help solve a problem they are widely believed to have exacerbated, praised the project’s collaboration of private companies, nonprofits, government agencies and community leaders.