Alphabet's 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 it had been offered $7.35 per share in cash, a premium of about 19% to the stock's closing price yesterday.

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

Fitbit has been a market leader in wearable fitness trackers, however it has faced growing competition in recent years, particularly from rival Apple.

"We have built a trusted brand that supports more than 28 million active users around the globe who rely on our products to live a healthier, more active life," Fitbit co-founder and chief executive James Park said in a statement by the two firms announcing the deal.

"Google is an ideal partner to advance our mission."