The 25-year-old can play as full-back, wing or centre and boosts Gloucester as injuries and the Six Nations are likely to deplete their squad.
Gloucester, who face London Wasps in the Heineken Cup quarter-finals in April, may register van der Bergh to compete in that match.
ERC rules state clubs involved in the knock-out stages may nominate two additional players, in any position by the registration date of March 19.
Director of rugby Nigel Melville told Gloucester's website: 'We're delighted to add Riaan to our squad.
'He has good experience and will provide us with a number of options in various back-line positions.'
Van der Bergh added: 'It's very exciting to be signing with Gloucester as so many people from around the world have heard of the club.
'I'm coming to Gloucester to prove to the team and to myself that I can play at the top level.
'The club has some quality players and the Zurich Premiership is one of the finest tournaments around.
'I'm looking forward to getting back to training and settling in with the rest of the players.'