The first-time study from Internet audience measurement company Nielsen/NetRatings found that 21 percent of those Web users now primarily use online versions of newspapers, while 72 percent still read print editions.
The remaining 7 percent split their time between online and offline editions. Comparable historic statistics were not available.
"A significant percentage of newspaper readers have transferred their preference from print to online editions," said Nielsen/NetRatings senior media analyst Gerry Davidson.
The statistics reflect a broad trend in the newspaper publishing industry.
U.S. newspaper circulation has been falling as audiences increasingly turn to the Internet for news--both on newspaper-run sites and through companiesand Google, which display news gathered from various sources.
Newspaper publishing revenue is growing slowly, but remains in a prolonged slump. Internet companies such as Google and Yahoo have been postingon a revival in more traditional online advertising, as well as fast-growing Web search ads.
In response to the increased traffic to news sites, online editions of newspapers often now include frequent news updates, original content, message boards and editorial blogs.
The online edition of the New York Times Co.'s flagship paper, NYTimes.com, is the most visited U.S. newspaper site. It had an audience of 11.3 million in May, up 25 percent from a year earlier, according to Nielsen/NetRatings.
USAToday.com had the second-biggest online reach, with an audience of 9.2 million in May, up 15 percent. Rounding out the top three was WashingtonPost.com whose audience in May grew 10 percent year over year to 7.4 million.
Visits to Yahoo News and Google News, which were not counted in the study, also have been on the rise.
In May, Yahoo News had an online audience of 23.8 million and the audience for Google News was 7.1 million, respectively up 3 percent and 4 percent from a year ago, according to Nielsen/NetRatings.