Wearable device sales are on track to grow 16.7% this year to 310.4 million, representing sales of $30.5 billion, Gartner has projected.
Nearly a third ($9.3 billion) of the total value of the market is expected to come from smart watches.
Gartner predicts there will be 45.1 million smartwatches sold this year, and by 2019 the devices are expected to be the second best selling category of wearable devices behind Bluetooth headsets.
While Apple is expected to continue to have the greatest market share of any smartwatch provider, its market share is expected to decrease from around a third in 2016 to a quarter in 2021 as more providers enter the market.
Gartner said Apple may announce an Apple Watch in September that will enable direct cellular connectivity for texting, interacting with Siri or transferring sensor data when a Wi-Fi network is not present.
"Smartwatches are on pace to achieve the greatest revenue potential among all wearables through 2021, reaching $17.4 billion," Gartner research director Angela McIntyre said.
“The overall ASP of the smartwatch category will drop from $223.25 in 2017 to $214.99 in 2021 as higher volumes lead to slight reductions in manufacturing and component costs, but strong brands such as Apple and Fossil will keep pricing consistent with price bands of traditional watches."