According to a new report by Gartner, the handset has grown 7.8% in the final quarter of 2019, thanks to "price reductions".
Gartner's research vice president Annette Zimmermann said: "A slight lowering of the prices of the iPhone 11 series, compared with the iPhone XR, and other price reductions for previous-generation iPhone models, increased demand."
The report adds that iPhone sales were strong in China, while areas such as France, Germany, the UK, Brazil and India also saw a boost.
It's expected that Apple could launch its first 5G phone in the third quarter of 2020.
Overall, global smartphone sales dropped 0.4% year over year in the final quarter of 2019, while sales as a whole declined by 1% over the entire year.
Anshul Gupta - senior research analyst at Gartner - added: "2019 ended a little better than expected, due to slightly improved performances from North America and emerging Asia/Pacific. "However, India, which sold 151.9 million units, overtook the US to become the No. 2 country for smartphone sales in 2019.
"China maintained the lead, with sales of 390.8 million smartphones over the full year."