The terms of the deal were not disclosed, but Bloomberg reported late yesterday that the purchase price is below $40 million.
Shares of Fitbit were rallying 1.06% to $8.07 in early-morning trading on Wednesday, but have nonetheless plunged nearly 73% so far this year.
The transaction includes personnel and intellectual property related to software and firmware development, but does not cover any of Pebble's hardware.
Pebble will separately sell off the remainder of its assets, sources told Bloomberg.
"With this acquisition, we're well positioned to accelerate the expansion of our platform and ecosystem to make Fitbit a vital part of daily life for a wider set of consumers, as well as build the tools healthcare providers, insurers and employers need to more meaningfully integrate wearable technology into preventative and chronic care," Fitbit CEO James Park said in a statement.
The acquisition is Fitbit's second this year. The maker of activity trackers purchased payments startup Coin in May to develop a near field communications (NFC) payment solution to embed into future Fitbit devices.