The value of the acquisition is expected to be somewhere between $34 million (£27 million) and $40 million (£32 million), VentureBeat reports.
Pebble has been backed by over $58 million (£48 million) in funding, so an exit of that size would be a disappointing outcome for the company and investors.
The company, which launched its first smartwatch in 2012, raised more than $40m of its overall funding through Kickstarter campaigns, including the crowdfunding platform's biggest campaign in 2015.
The acquisition, first reported by The Information, will reportedly see Pebble's intellectual property (IP) and staff transferred to Fitbit, possibly helping it to develop new products. However, the Pebble brand is expected to be phased out.
It is currently unclear when the deal will go through.
Pebble, which announced it was laying off 25% of its employees in March, has been searching for a buyer for some time, according to The Verge.
The company's smartwatches, which come with their own operating system, retail for between £70 and £200, but the smartwatch category is yet to really take off, even though it was once being hailed as "the next big thing" in consumer technology.
At the time, Pebble CEO Eric Migicovsky said the company has raised $26 million (£21 million) in the eight months prior to the interview on top of its $20 million (£16 million) Kickstarter campaign that started in February 2015. "We've definitely been careful this year as we plan our products," Migicovsky said. "We got this money, but money [among VCs in Silicon Valley] is pretty tight these days."
Fitbit became a publicly listed company in June 2015 when it IPO'd on the New York Stock Exchange at $20 (£16) a share, giving it a valuation of $4.1 billion (£3.3 billion).
Fitbit did not immediately respond to Business Insider's request for comment.