The stock resumed trading and ended the day up 30.5%, adding more than $330 million to its market cap to bring it to around $1.5 billion. Fitbit shares are now up more than 12% for 2019.

Google parent company Alphabet made an offer to acquire Fitbit , a source familiar with the matter told CNBC Monday. Reuters first reported that Alphabet made the offer to buy Fitbit. Fitbit's stock was halted after skyrocketing more than 18%.

The deal would make Alphabet a player in the wearable fitness tracking space, competing against the likes of Apple, which recently released a new version of its popular smartwatch. Google licenses its Wear operating system to companies such as Fossil but does not currently make its own smartwatch.

Google has described its hardware strategy as "ambient computing," meaning users should be able to access its services wherever they are. Buying Fitbit could be a play to make Google services a greater part of customers' lives and measure up to Apple in the health and fitness space. Google hired former Geisinger Health CEO David Feinberg last year to consolidate its health-care strategy. The company announced several new hardware products earlier in October, including the new Pixel 4 smartphone.

For Fitbit, support from Alphabet could grant a much-needed boost for the company, which has seen Apple take over about half of the global smartwatch market in 2018 in terms of units shipped, according to Strategy Analytics. In its July earnings release, Fitbit lowered its guidance for the year, citing weaker-than-expected sales of its new lightweight watch.

Fitbit declined to comment. Alphabet did not respond to requests for comment. Alphabet's CFO declined to comment on the potential Fitbit acquisition in an interview following Alphabet's earnings report Monday afternoon.

--CNBC's Josh Lipton contributed to this report.

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