San Francisco e-cigarette company Juul is in talks to buy a downtown office tower, even as the fast-growing startup faces scrutiny from city officials for popularizing youth vaping.

Juul is nearing a deal to buy 123 Mission St., a 29-story high-rise in the Transbay area, according to three people familiar with the company’s plans.

The 29-story tower last sold for $290 million in 2018.

Juul spokesman Josh Raffel declined to comment on specific real estate deals. He said the company is growing rapidly, going from 200 to 2,000 employees in 2018. A large portion of its workforce is in San Francisco.

“As a result, we are currently looking for additional office space in San Francisco and the surrounding Bay Area, but we have nothing to announce at this time,” Raffel said.

Nearly 40 percent, or 153,000 square feet, of the 387,598-square-foot 123 Mission St. tower is available for lease, according to brokerage data — room for around 1,000 employees. Additional space will be available next year, when a Salesforce Foundation lease expires.

The entire tower is five times as large as Juul’s current headquarters of 76,000 square feet in a renovated historic building at Pier 70. Juul also recently expanded in Mountain View and in Austin, Texas.

Juul is poised to have a vast amount of cash to fuel its expansion. Malboro maker Altria agreed to invest $12.8 billion in Juul in exchange for a 35 percent ownership stake, but the deal has yet to be approved by regulators. Juul has separately raised around $800 million in additional venture funding, according to Crunchbase.

Juul plans to remain headquartered at Pier 70, a historic shipyard owned by the city. The company’s presence there sparked backlash from residents who say Juul is harming public health because vaping has become so popular among teens.

That prompted San Francisco City Attorney Dennis Herrera and Supervisor Shamann Walton to introduce legislation that would prevent e-cigarette companies from leasing public land in the future. Herrera and Walton also proposed legislation that would halt the sale of e-cigarettes altogether in San Francisco until federal regulators approve the products through a process called a pre-market review — which they have yet to do for any e-cigarettes currently on the market.

Juul has said its aim is to help adult cigarette smokers transition to vaping, which researchers say is less toxic than cigarettes. It also says it has stopped the shipment of flavored pods to retail stores in an effort to curb youth sales.

Northwood Investors, a New York company, bought 123 Mission St. last year from HNA Group, a Chinese conglomerate that has sold billions of dollars in U.S. real estate amid debt problems. Northwood didn’t respond to a request for comment.

The potential deal would be one of the largest property acquisitions by a San Francisco company not specialized in real estate. Firms predominantly sign leases rather than purchase buildings, with a few exceptions.

In February, the Gap bought 550 Terry Francois Blvd., the headquarters of its Old Navy division, for $342 million. In 2015, Salesforce paid $637 million for 50 Fremont, which is across the street from Salesforce Tower. In 2014, Google bought 188 The Embarcadero for $65 million. Uber owns a portion of its new four-building Mission Bay headquarters, and also bought and sold Uptown Station in Oakland.

Roland Li and Catherine Ho are Chronicle staff writers. Email: roland.li@sfchronicle.com, cho@sfchronicle.com Twitter: @rolandlisf, @Cat_Ho