The 27-year-old Welshman has made over 60 appearances for Ospreys since joining the club from Newport Gwent Dragons in 2009.
"He's a very difficult guy to stop, really hits some hard lines, breaks tackles and is a big hitter defensively," director of rugby Nigel Davies told Gloucester's official website.
"I know speaking to a lot of the players down at the Ospreys that they rate him very highly.
"Having watched him play for a long time, I have a lot of faith that he'll come and make a big impact and really put pressure on the incumbents."
Gloucester continue their Aviva Premiership campaign on Saturday when they make the trip to Exeter Chiefs.