The company, which will release its official results next month, is expecting 2017 to be more challenging than the year before, "as new products introduced in the first half of 2016 comprised more than half of revenues". It predicts revenues of $1.5 billion to $1.7b in 2017.
It remains optimistic, however, and said the results are a reflection of a "temporary slowdown" in the wearables market.
"We are confident this performance is not reflective of the value of our brand and company's long term potential," said James Park, Fitbit's chief executive. Last year Fitbit launched its own smartwatch, the Blaze.
The company, which has 23.2m active users and a 75 per cent share of the market, acquired its main rival Pebble last year in a bid to cement its position as market leader.