In the second quarter of 2015, Apple shipped 3.6 million units of the Apple Watch globally, just under Fitbit's 4.4 million units, according to a report on wearable devices from the International Data Corporation (IDC).

Fitbit still has the largest market share, 24.3 percent, though its share is 6 percent less than it was in the second quarter of 2014. Meanwhile, Apple, which only began shipping its device this year, had 19.9 percent market share in the second quarter. IDC added that the total shipment volume of wearables for the second quarter reached 18.1 million units, which is 223.2 percent more than the number of units shipped in the same quarter of last year, 5.6 million units.

"Anytime Apple enters a new market, not only does it draw attention to itself, but to the market as a whole," IDC Research Manager Ramon Llamas said in a statement. "Its participation benefits multiple players and platforms within the wearables ecosystem, and ultimately drives total volumes higher. Apple also forces other vendors – especially those that have been part of this market for multiple quarters – to re-evaluate their products and experiences. Fairly or not, Apple will become the stick against which other wearables are measured, and competing vendors need to stay current or ahead of Apple."

After Apple, IDC reported that Xiaomi shipped 3.1 million units and had 17.1 percent market share, Garmin shipped 700,000 units and had 3.9 percent market share, and Samsung shipped 600,000 units and had 3.3 percent market share. The total number of units shipped for all other devices totaled 5.7 million and these other devices had 31.5 percent market share. Notably, Jawbone's marketshare wasn't big enough for IDC to break it out out of the other category.

In the first quarter of 2015, IDC reported in June, approximately 11.4 million wearables were shipped, which was a 200 percent increase from the 3.8 million wearables shipped in the first quarter last year. Fitbit had 34.2 percent market share, down from 44.7 last year in the first quarter. At the time, following Fitbit was Xiaomi, which had 24.6 percent market share even though the company had just started shipping devices during the second half of 2014.