But sales for Chinese vendor Huawei went in the opposite direction, rising 23% year-on-year, Reuters reported.
Apple's share of the market fell from 12.5% a year ago to 11.5% in 2018, the data showed.
The company's own results have shown a drop in revenue for the device as well, with revenue for the first quarter of its financial year falling by 15% year-on-year.
“The severe macro environment in China and the assault of domestic brands’ innovative products have also been reasons for Apple’s continued decline.”
Another analyst firm, Counterpoint Research, said iPhone sales had fallen in India as well.
The firm said iPhone sales in the fourth quarter fell 25% year-on-year, meaning that the number of units sold fell to 1.7 million from 3.2 million a year earlier.