Threat Stack, a cloud security company based in Boston’s Fort Point, will move to a new office in Downtown Crossing as it triples its space to accommodate growth.

The company signed a seven-year lease for 13,768 square feet at 55 Summer St., owned by Boston-based Synergy Investments and DivcoWest.

In a reflection of the neighborhood’s increasingly sought-after office market, it had to beat out a group of other tenants submitting proposals to move into the former Yahoo offices across the street from Macy’s. The three finalists were invited to make pitches to ownership.

“We literally went in and presented why Threat Stack would be a great tenant for 55 Summer St.,” CEO Brian Ahern said. “My suspicion is there’s a lot of interest in the building and it just speaks to the demand in the city and this space.”

Yahoo will move out of two floors in the building on June 30 and Threat Stack will occupy the entire second floor with an option to expand, Ahern said. The market-rate deal included a generous tenant improvement package, he said. Threat Stack will remove the vestiges of Yahoo’s presence, including its purple-and-yellow color scheme, and install new furniture. The space lends itself to tech offices with oversized window lines and 18-foot ceilings, Ahern said.

Threat Stack also looked at sublease space in the Seaport District, but none offered tenant improvement packages and the terms were longer, Ahern said.

“The pricing in the Innovation District is escalating. We felt moving over to Downtown Crossing was in our best interests for affordability, accessibility to retail and hospitality and the expansion potential,” he said.

Threat Stack opened its offices at 266 Summer St. in Fort Point in February 2015.

After receiving an infusion of $15.3 million in series B funding led by Scale Venture Partners in April, Threat Stack plans to increase its Boston workforce from 40 to 70 employees by year’s end. Employees will relocate in early August.

Synergy and DivcoWest bought the 10-story, 125,000-square-foot building in 2014 for $48 million.

Office vacancies in peripheral downtown submarkets including Downtown Crossing, North Station and South Station have dropped from 9.1 percent to 5.8 percent in 12 months, according to a first-quarter report by Boston-based Encompass Real Estate Strategies.

Downtown Crossing had 164,000 square feet of positive absorption during the first quarter, bringing the vacancy rate down to 10 percent. Class A rents averaged $48 per foot, compared with $61.47 in the Seaport.

Tech companies seeking transit-friendly offices have accounted for much of the recent leasing activity. In March, SimpliSafe expanded by 20,271 square feet at 294 Washington St. just over a year after relocating from Cambridge. The company makes wireless home security systems that customers can monitor on their smartphones.