"Wi-Fi in our city's parks is another step toward a larger vision of connectivity for our City as a whole, bridging the digital divide and ensuring that our diverse communities have access to innovation," Mayor Ed Lee said in a statement.
Internet giant Google donated about $600,000 to help the city buy and install Wi-Fi equipment and cover maintenance cost for two years, according to the San Francisco Chronicle.
(Also see: Google offers to fund wireless hotspots in San Francisco)
"This network will make the web more accessible than ever for thousands of our neighbor - getting online is as easy as heading to the local park," Rebecca Prozan, Google's public policy and government affairs manager, said in a statement.
Union Square, Balboa Park, Alamo Square and Children's Tenderloin Rec Center are among the places where people can log on by choosing the "(hash)sfwifi" option on their smartphones, tablets or laptops.
Many communities already have pockets of limited free Wi-Fi, but until now those services are mostly either centered around public hotspots like parks, hospitals or libraries or offered by major tech firms like Google, which provides Wi-Fi throughout its headquarters city of Mountain View.
Google also recently rolled out New York City's biggest contiguous free public Wi-Fi in the Chelsea neighborhood, where the company has a campus.
City officials have been talking about providing free Wi-Fi citywide for the past seven years, but the rollout has been slower than anticipated.
Free access is already available in parts of City Hall, public libraries, San Francisco International Airport and along the Market Street corridor.
The free Internet initiative will help many low-income students who need Internet access to do online homework assignments, and parents can use email more easily to communicate with the school, said Rodney Chin, executive director of the Buchanan YMCA.
"It's a step toward leveling the playing field," Chin told the Chronicle.
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