Amazon.com Inc. said it’s hiring 2,000 people in Boston, expanding an existing technology hub that focuses on everything from machine learning and speech science to cloud computing and robotics.

The Seattle-based e-commerce giant plans to open a 430,000-square-foot (39,900-square-meter) office in the city’s Seaport waterfront development by 2021, bringing the company’s total headcount in the Boston area to more than 3,500 full-time employees, Amazon said in a statement Tuesday.

“In just a few years, we’ve grown from a handful of software developers and scientists to a team of more than 1,200, inventing new capabilities and products on behalf of millions of customers around the world,” said Rohit Prasad, Amazon’s Boston area-based vice president and head scientist of Amazon Alexa.

Amazon’s current Boston technology employees mostly work on Alexa, Amazon Web Services and Audible, the company said. Amazon employed about 566,000 full- and part-time employees worldwide at the end of December, and on Monday announced it was hiring 3,000 people in Vancouver.

Boston is among a list of 20 finalists to land Amazon’s second headquarters, which would create about 50,000 new jobs and cost more than $5 billion.