Rabeni, who was a Guinness Premiership winner with Leicester in 2007, will move to Kingsholm at the start of next season.
"I am delighted to be joining Gloucester although it was a very tough decision to make, after five years at Welford Road," said Rabeni.
"It is now time for me to move on to another challenge, and hopefully help Gloucester to lift some silverware, continue the good work and progress the team has so far achieved.
"I do feel that I have some best years of my rugby left and would like to give everything to Gloucester and help the young players coming through with the experiences that I have gained at Leicester and with Fiji.
"There are some great players at the club and it will be an honour and privilege to work and fight with them in Gloucester's cause."
Before arriving in England, the 30-year-old centre played Super rugby with the Highlanders in New Zealand.
He has two World Cup campaigns behind him, having helped Fiji to the quarter finals in 2007 after making his World Cup bow in Australia four years earlier.
With Gloucester having been the nearly men of English rugby in recent seasons, head coach Ryan believes Rabeni's experiences at the top level can help his side finally get their hands on some silverware.
"Seru's signing is great news for us, particularly at this point in the season" said Ryan.
"He is a player of great quality and physicality who is able to play both in the centre and on the wing - that versatility and familiarity across positions can only strengthen our options.
"His presence within the side will help strengthen our position within Guinness Premiership next season and his expertise and experience playing in knock-out stages will be key going forward."