That was down from 26.7 per cent of smartphones sold in the fourth quarter of last year, which included the holiday shopping season, IDC found.
The iPhone, however, has only been available on one US carrier - AT&T.
Much of the slack was picked up by Research In Motion's BlackBerry, which took 35.1 per cent of the market in the fourth quarter and then 44.5 per cent in the first.
IDC analyst Ramon Llamas said the BlackBerry is now strong in the "prosumer" segment, as RIM has successfully widened the appeal of the device beyond the professionals who have been its core customer group.
Smart phones are designed for web surfing and email in addition to voice calls and usually have alphabetic keyboards or touch screens. They account for a growing share of mobile phones sold, as prices descend and carriers complete their fast data networks.
IDC did not reveal the total number of smart phones sold in the quarter. Apple said it sold 1.7 million iPhones in the first quarter, including overseas sales.
Palm, a pioneer in the category along with RIM, also picked up market share in the first quarter, when it grabbed 13.4 per cent of smart phone sales, up from 7.9 per cent in the fourth quarter, IDC said.
"Palm also did really well. It posted a sequential gain mainly on the strength of the Centro phone," Llamas said. The Centro, a smaller phone than Palm's Treo models, came out last fall for the Sprint Nextel network and was launched by AT&T Inc. in February.
But Palm's market share is down from 23 per cent in the first quarter a year ago, apparently the victim of the iPhone, which went on sale late last June. RIM's market share is also down from last year, when it sold 48.7 per cent of U.S. smart phones in the first quarter.
No. 4 in market share in the most recent quarter was Samsung Electronics, with 8.6 per cent, up from 5.1 per cent in the fourth quarter, a rise Llamas credited to the availability of the BlackJack on Verizon Wireless.
Motorola, which is struggling in the overall mobile phone market, did poorly in smart phones as well, dropping from a 7.5 per cent share in the fourth quarter to 2.6 per cent in the first.