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Move over Manhattan: Office rents in San Francisco have eclipsed those in New York’s costliest borough as the most expensive in the country.

A combination of the booming technology sector and tight supply of commercial real estate propelled average office rents that San Francisco landlords are asking for to $72.26 a square foot in the fourth quarter of last year, edging out the $71.85 a square foot in Manhattan, according to a new report from CBRE Group, a commercial real estate services and investment firm. The prices in San Francisco rose 14 percent last year, compared with 7 percent in Manhattan.

Not since the original Internet gold rush more than a decade and a half ago has the price of office space in San Francisco surpassed that of Manhattan, long the country’s most expensive commercial market. San Francisco’s supremacy back then proved short-lived, as the bursting of the dot-com bubble in 2000 wiped out many start-ups. The era of social media and sharing economy companies has brought a flood of technology companies back to San Francisco, including icons like Twitter, Uber and Airbnb.

Office rents could tumble again if the tech sector falters badly, and there are anecdotal signs that some companies are struggling. Sidecar, a San Francisco-based ride-hailing start-up, last month said it would stop offering its services. Investors have lowered the valuations of several so-called unicorns, private technology companies originally valued at over $1 billion.

Yet evidence of a more cataclysmic shakeout in tech hasn’t materialized yet, and until it does, the commercial real estate market is likely to keep humming along.

In a phone interview, Colin Yasukochi, director of research and analysis at CBRE, said about 29 percent of the occupied office space in San Francisco is filled with tech tenants, around double the percentage back during the height of the first dot-com boom. The growth in the tech sector overshadows all else in the city.

“The market here in San Francisco has been largely driven by the tech industry, which has really been one of the growth leaders coming out of the recession,” Mr. Yasukochi said. “Manhattan is more dominated by financial services firms who were not on that same growth trajectory coming out of the recession.”

The soaring cost of office space in San Francisco has added to other pressures technology companies in the city are feeling, including increasingly expensive housing. Rents for a one-bedroom apartment in San Francisco are the highest in the country, according to Zumper, an online apartment rental firm.

Few technology companies are abandoning the Bay Area entirely because of the abundance of talent and venture capital there, instead choosing to open satellite offices out of state or to relocate to less expensive nearby cities. One such start-up, 99Designs, an online marketplace for designers, relocated to Oakland from San Francisco about six months ago.

Patrick Llewellyn, the chief executive and president of the company, said many of the start-up’s employees lived in the East Bay, where Oakland is located, and the company was able to find a far better space in the city’s Uptown neighborhood than it could in San Francisco. He said 99Designs could have afforded to pay the higher office rents in San Francisco, but the terms were onerous.

“Landlords are demanding seven-year leases at minimums,” Mr. Llewellyn said. “In the fast-moving landscape that is the start-up business, that’s a long, long time.”

It’s important to note that CBRE’s rankings are based on average rents, and one of the unique characteristics of the San Francisco commercial real estate market is how little variation there is between office prices in upscale and less-upscale neighborhoods, compared with markets like Manhattan. Mr. Yasukochi attributes that to a more flexible attitude toward workplaces among technology start-ups, which tend to be more concerned about space for bicycle parking and dog play areas than striking views.

In nearby Silicon Valley, there is also far greater variation in commercial rents than in San Francisco. The average rent in Silicon Valley as a whole is $52.92 a square foot, according to CBRE, significantly less than the average in San Francisco. But that figure encompasses towns like Milpitas, at around $20 a square foot, and the chic streets of Palo Alto, near the epicenter of venture capital and Stanford University, where the average office rent is about $100 a square foot.

The chart-topping office rents in San Francisco are also a testament to the city’s geographic constraints, crammed as it is into 47 square miles at the top of a peninsula. The technology scene to the south in the sprawl of Silicon Valley is now and will most likely remain far bigger.

Of the 172 million square feet of office space in the Bay Area occupied by technology companies, only about 13 percent is in San Francisco, with around 73 percent of it in Silicon Valley, CBRE estimates.