Wearable device maker Fitbit Inc reported a surprise third-quarter adjusted profit on Wednesday, driven by its fast-growing smartwatch business that contributed to nearly half of its total revenue.

The San Francisco-based company's shares jumped nearly 9 percent to $5.15 in after-market trading.

Fitbit sold 3.5 million devices in the reported quarter, beating estimates of 3.42 million, according to four analysts polled by FactSet. Average selling price increased 3 percent to $108 (84.5 pounds) per device.

The company, which is popular for its colourful fitness trackers, is a late entrant to the smartwatch market and has been facing stiff competition from tech players with deeper pockets such as Apple Inc and Samsung Electronics Co Ltd.

Fitbit now expects fourth-quarter revenue to be more than $560 million, compared to average analysts' estimate of $569.2 million, according to Refinitiv data.

The company's net loss narrowed to $2.1 million, or 1 cent per share, in the quarter ended Sept. 29, from $113.4 million, or 48 cents per share, a year earlier.

On an adjusted basis, the company earned 4 cents per share. Revenue rose marginally to $393.6 million from $392.5 million.

Analysts on average had expected the company to lose 1 cent per share, on revenue of $381.2 million.