A total of 21 million wearable devices were sold in the third quarter of the year, up almost 200% on the same period last year, according to research company IDC.
Over a fifth of these were sold by Fitbit, which moved 4.7 million wearable products between July and September, IDC said, capturing 22% market share.
Although Apple has not revealed sales of its Watch, IDC estimates 3.9 million were sold in the period, putting it in second place just one quarter after it first arrived on the market.
Chinese company Xiaomi experienced a strong quarter, capturing 17% of the market, putting it in third place and securing it an 815% rise in market share compared with the same period last year.
The rapid growth in wearable sales at Xiaomi and at other Chinese vendors like XTC reflects the reality that China has emerged as the fastest-growing wearables market globally, IDC said.
The company said a strong opportunity exists for manufacturers making cheaper smart watches, as the average price of such wearables is $400, compared to $94 for an average basic watch.
Fitness trackers continue to perform well in the wearable markets, despite the growing attention around smart watches.
Meanwhile, IDC predicted that 2015 would be the first full year in which smartphone sales worldwide would only experience single-digit percentage growth.
It is expecting shipments to grow 9.8% in 2015 to a total of 1.43 billion units, with slower-than-expected growth in China, Latin America and Western Europe.
