The network covers a three-mile stretch from Castro Street to the Embarcadero.
Mayor Ed Lee says a 250,000 people will benefit from the free Wi-Fi service every day.
Bay Area-based "Ruckus Wireless" is paying for the $700,000 project.
Google is also giving the city a $600,000 grant to pay for Wi-Fi installation in 31 parks and plazas citywide, starting next summer.