Google will expand its San Francisco offices in one of the city’s largest deals of the year, underscoring tech’s seemingly endless appetite for expansion.

The company said it has leased 300,000 square feet — room for more than 1,500 employees — at the Landmark at One Market building, which was formerly the headquarters of Salesforce.

Salesforce formally changed its headquarters to Salesforce Tower on Nov. 1, according to corporate filings.

The deal raises Google’s office space in San Francisco by 30 percent to 1.3 million square feet, according to data from brokerage Cushman & Wakefield. Google is the third-largest tech tenant in San Francisco, behind only Salesforce and Uber, according to the data. Yet San Francisco is only about 5 percent of the Mountain View company’s entire Bay Area holdings, which total 20 million square feet, or the equivalent of 14 Salesforce Towers.

Big tech deals pushed San Francisco’s office market to record-high rents this year. In the past 12 months, Google, Facebook and Amazon all have signed new leases or expanded existing offices in the southern part of the Financial District, which includes the area around the new Transbay transit center named after Salesforce. Younger, well-funded tech companies such as DoorDash, Twilio and SoFi have also expanded by millions of square feet in the city this year. The 1916 Landmark at One Market building was previously the headquarters of Southern Pacific Railroad. It was later connected to two towers on Spear and Steuart streets, buildings where Google is already a tenant.

The company’s other San Francisco locations include 345 Spear St., 121 Spear St., 2 Harrison St. and 188 The Embarcadero, according to Cushman & Wakefield.

American Assets Trust owns the Landmark. The firm did not respond to requests for comment.

Google is growing around the world. The company is reportedly in talks to occupy 1.3 million square feet in Manhattan, according to the New York Post. The company also expanded in Chicago and signed a small deal in Tel Aviv.

In the Bay Area, Google has major plans for new offices in Mountain View’s North Bayshore district and in San Jose near Diridon Station.

At the same time that they expand locally, Bay Area tech giants and startups have also been growing out of state as they seek different talent pools and cheaper markets.

Editor’s note: This story has been updated to correct the owner of the Landmark.

Roland Li is a Chronicle staff writer. Email: roland.li@sfchronicle.com Twitter: @rolandlisf