Monthly unique browsers for the news outlet topped 29.3 million last month, an increase of 29.6 per cent on April and 198.2 per cent on May 2013.
Metro relaunched its website under a mobile-first strategy at the end of 2012, and although it initially brought a drop in traffic to the website, the number of unique monthly browsers has almost quadrupled since.
Other notable month-on-month increases included the Daily Star, where month-on-month web traffic grew 27.2 per cent, to 8.7 million unique browsers, and The Express, up 10.7 per cent to 11.9 million uniques.
Both outlets, owned by Express Newspapers, have only been including figures in ABC's monthly multi-platform report since February 2014, but have been the only two publications to report consistent growth in traffic figures over that period.
Meanwhile, The Independent also saw a 6.9 per cent increase to 1.6 million unique browsers.
Monthly increases for Mail Online, the Guardian and the Telegraph were negligible, at 3.8 per cent, 3 per cent and 2.2 per cent respectively.
Mirror Group Nationals were the only outlet to see a drop in traffic, with a slight decrease of 0.9 per cent since April, to 47.75 million.
However, with the exception of Metro, the Mirror Group Nationals has seen the biggest year-on-year increase in traffic of all other audited national news sites at 81.8 per cent.
Additional reporting by Alastair Reid.