While the expansion is not Amazon’s so-called second headquarters — that prize is still being sought by Boston and other cities around the country — the project has the potential to make Boston one of Amazon’s largest outposts beyond the company’s Seattle base, and further boost the city’s already robust tech economy.

According to documents filed with the city this week, Amazon is in negotiations to rent a building in the planned Seaport Square project along Congress Street, with an option to lease a second neighboring building for more growth. Developer WS Development said it could start construction on the first building this summer, and have it ready in 2021.

Amazon is poised to lease a massive new office in the Seaport District and hire at least 2,000 more employees in Boston over the next few years, Mayor Martin J. Walsh said Tuesday. If the deal closes as expected, it would nearly triple the e-commerce giant’s workforce in Boston and Cambridge, and make the company one of the region’s biggest private employers.

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If Amazon fills both buildings, it would be able to add as many as 4,000 jobs in Boston over the next several years in the heart of the booming Seaport District. Most of the well-paying positions likely would be in technology-related areas such as robotics, cloud computing, and voice-activated systems.

“Amazon is considering additional options for expansion in the Boston area to meet the demand of our growing employee base and as part of our ongoing hiring efforts,” the company said in a statement that also stressed the expansion plans are “independent” of Amazon’s search for a so-called second headquarters. Boston is one of 20 finalists for that coveted project.

The Seaport Square deal is not expected to be finalized for several weeks, said people familiar with it. The plans were outlined in a filing Monday by WS to the Boston Planning & Development Agency. Walsh formally announced the project Tuesday evening during a downtown business leaders meeting.

“It’s not HQ2. But it is great news for Boston,” Walsh said. “Projects like these are a tremendous vote of confidence in our city’s economy.”

As part of the deal, WS Development and Amazon are seeking $5 million in city property tax breaks, which would be contingent on the company creating 2,000 new jobs within 25 miles of Boston by 2025. That radius would include Amazon’s offices in Kendall Square and a robotics plant in North Reading, but not its massive warehouse in Fall River. The tax break would double to $10 million if Amazon creates another 2,000 jobs in the second building.


It was not clear Tuesday whether Amazon also is seeking state incentives. A spokeswoman for Governor Charlie Baker confirmed that state officials have been talking with Amazon but would divulge no details until a deal is finalized.

Walsh signaled his support for the city tax breaks, which must be approved by the BPDA board,.

“This project filing is the first step, and we’ll be working with Amazon to make it a reality,” he said.

Since opening a small office in Cambridge’s Kendall Square about five years ago, Amazon has been on a growth tear in the region. Today, it employs about 1,200 people in Kendall and a temporary office in Back Bay. Last year, Amazon announced plans for a 900-person office in Fort Point, which is on track to open this year.

The company, which employs more than 500,000 people nationwide, has been on a hiring spree in recent years, both at its massive warehouses and at “tech hubs” in cities across the country, most of which are also finalists for the second headquarters. It has tripled its head count in Austin, Texas, in recent years, and has grown fast in Atlanta. Earlier this month it announced an expansion in Pittsburgh and last fall it unveiled plans for 2,000 new hires in Manhattan.


Their search for a big office in the Seaport has been widely rumored in Boston real estate circles in recent months. The WS filing is a sign that a deal is imminent, though the documents note that the two sides are still “negotiating.”

Under the proposed deal, Amazon would occupy 16 stories — 430,000 square feet — of an 18-story building that was recently permitted along East Service Road. Amazon would also have an option to lease a second building in the same project, at the corner of Congress Street and Boston Wharf Road, potentially giving it more than 1 million square feet of office space in all.

A 2,000-person office would make Amazon the largest employer in the Seaport, which has become a hotbed of blue-chip companies. Earlier this month, insurance giant MassMutual announced plans for a big new building there, joining recent newcomers such as Boston Consulting Group, PricewaterhouseCoopers, and tech firm PTC. Several large housing developments are newly opened or under construction as well.

All the activity is heightening worries about traffic, though developers and transportation officials are at work on a variety of solutions. WS, for instance, has agreed to invest $20 million in streets, bike lanes, and a new Silver Line entrance at Seaport Square. Either way it’s a sign of strong demand, said Brendan Carroll, director of intelligence at real estate firm Perry Brokerage Associates.


“It’s pretty tremendous,” he said. “We’re seeing the Seaport District develop in a fashion faster than anybody could have imagined. It’s starting to equal the Back Bay in terms of size and commercial activity.”

It’s not clear how the move might impact the even-bigger prize of Amazon’s HQ2.

The company is six months into a nationwide search for a second home to complement its Seattle home, seeking a site for an $8 billion campus that could someday employ 50,000 people. Boston has been considered a strong contender and in January was named — along with Somerville — one of 20 finalist cities, with a decision expected later this year.

While Amazon says officially that the Seaport expansion has nothing to do with HQ2, real estate analysts say it’s unlikely that any company — even one as unconventional as Amazon — would locate 4,000 people in one neighborhood and then build a headquarters campus somewhere else within the same city. That could signal an HQ2 in the Seaport, Carroll said, or mean that HQ2 will go to another city, with a huge office offered as a consolation prize for Boston.

“It’s really hard to tell what they’re up to,” he said.

Tim Logan can be reached at tim.logan@globe.com. Follow him on Twitter at @bytimlogan.