Alphabet Inc-owned Google will buy wearable device maker Fitbit Inc for $2.1 billion, as the search giant looks to enter the fast-growing market for fitness trackers and smartwatches.

Fitbit said on Friday it had been offered $7.35 per share in cash, a premium of about 19% to the stock's closing price on Thursday.

Fitbit shares have gained more than 40% since Reuters exclusively reported on Monday that Google had made an offer for the maker of the popular colourful fitness trackers.

While Google has joined other major technology companies such as Apple Inc and Samsung Electronics Co Ltd in developing smart phones, it has yet to develop any wearable offerings.

Qatalyst Partners LLP acted as financial adviser to Fitbit, while Fenwick & West LLP was its legal adviser.

- Reuters