The market tracker said global smartphone sales will see a rise of 11.3 percent in 2015, following robust growth in 2014 of 27.6 percent.
IDC said the overall market is still moving forward thanks to growth in other countries and estimated sales rising to 1.9 billion units by 2019 from a projected 1.44 billion this year.
"Smartphone volume still has a lot of opportunity in the years to come, but two fundamental segments driving recent years' growth are starting to slow," said analyst Ryan Reith.
"As reported earlier in May, smartphone shipments in China actually declined year over year in the first quarter of 2015, showing that the largest market in the world has reached a level of maturity where rapid growth will be harder to achieve."
(Also see: China's Phone Makers Look to India for Growth)
Reith said China has been a key driver of Android, accounting for 36 percent of total volume in 2014.
Growth for Android, the free operating system from Google, will be only around 8.5 percent this year due to the China slowdown, according to IDC.