The company said it projected revenue of between $420 million and $440 million in the quarter. Analysts had on average expected $483.8 million, according to StreetAccount.
However its full-year 2016 earnings guidance of between $1.08 and $1.20 per share and sales of $2.4 billion to $2.5 billion came in largely in line with expectations, according to StreetAccount.
Shares of Fitbit have plunged more than 40 percent this year amid concerns about increasing competition. The company's stock in the regular session Monday traded near $16 per share, well below the $20 per share price from its initial public offering last year.
Fitbit said it sold 8.2 million devices in the fourth quarter, up from 5.3 million in the prior-year quarter. Its unit sales nearly doubled for the full year to 21.4 million.
The company said the global launch of two new products would hit its bottom line in the current quarter. It expects to see "higher sales and marketing expenses" as it rolls out Alta, a fashion-focused product, and Blaze, a smartwatch.
FItbit also expects "additional manufacturing costs" for the products. It said the timing of shipments for the devices could lead to "the majority of reorders" coming in the second quarter.
— CNBC's Everett Rosenfeld contributed to this report.