The 24-year-old has moved to Oakwell after his two-year deal with Oldham expired earlier this summer.
Mvoto, who had also been a target for Doncaster, opted to join the Tykes following talks with manager David Flitcroft who impressed him with his vision for the Championship club.
He told Barnsley's official website: "I'm very happy to be in Barnsley now. I heard about the move a couple of days ago.
"I had a talk with the manager. I like his project and everything he said. I just wanted to come to Barnsley."
Mvoto joined Oldham from Sunderland in 2011 after an earlier loan spell with the Latics and he made a total of 123 appearances for the club.
He did not feature for Sunderland after joining them from Paris St Germain in 2008 and also spent six months on loan with Southend during the 2009/10 season.