It was followed by the Samsung Galaxy S9 Plus, its 6.2-inch flagship smartphone, which shipped 5.3 million units in the first quarter. It was only on sale however for a short time in the quarter, making Apple's iPhone X the major high-end rival on the market.

Xiaomi, which filed for an initial public offering on Thursday , saw its Redmi 5A low-priced device ship 5.4 million units.

Apple shipped 16 million units of the iPhone X in the three months ended March 31, according to a study released by Strategy Analytics Thursday. It captured 4.6 percent market share of all models shipped in the quarter. The iPhone X was closely followed by Apple's iPhone 8 and iPhone 8 Plus which shipped 12.5 million and 8.3 million units, respectively. And then the iPhone 7 with 5.6 million units.

The iPhone X became the best-selling smartphone model in the first quarter of 2018, a new report shows, providing more good news for Apple investors following a better-than-expected earnings report earlier this week.

"For the second quarter running, the iPhone X remains the world's most popular smartphone model overall, due to a blend of good design, sophisticated camera, extensive apps, and widespread retail presence for the device," Juha Winter, senior analyst at Strategy Analytics, said in a press release.

The report from the analyst firm appears to back up the optimism Apple CEO Tim Cook had about the iPhone X during an earnings call earlier this week.

"Customers chose iPhone X more than any other iPhone each week in the March quarter, just as they did following its launch in the December quarter," Cook said.

Apple reported earnings Tuesday that beat expectations, despite bearish sentiment going into the event.

There are still voices that suggest that Apple could be winding down production of the iPhone X with the aim of scrapping it for new versions of the device. Several suppliers' earnings, that are key to the iPhone X supply chain, have pointed toward lower production, an analyst told CNBC earlier this week.

Meanwhile, Samsung's Galaxy S9 and S9 Plus were launched in February but didn't go on sale until mid-March, so they didn't have a full quarter to make an impact. But Strategy Analytics said the devices "are already off to a very good start."

"We expect the S9 Plus to become the best-selling Android smartphone globally in the second quarter of 2018," Woody Oh, director at Strategy Analytics, said in a press release.