The New York Times website had more than 32 million visitors and 719 million page views in May, with the average visitor to the site viewing 22 pages of content.
A short way behind was Tribune Newspapers, with 24.8 million visitors.
Jeff Hackett, comScore senior vice president, says the numbers prove online news is the future.
“The good news for publishers is that even as print circulation declines, Americans are actually consuming as much news as ever – it’s just being consumed across more media,” he said. “The internet has become an essential channel in the way the majority of Americans consume news content today with nearly three out of five internet users reading newspapers online each month.”