The 32-year-old Fiji captain moved to Vicarage Road in 2004 from Brisbane Easts and has made 65 appearances since.
Rauluni was one of the star players of last year's World Cup, inspiring his country to the quarter-finals.
Director of rugby Alan Gaffney said: "Moses was one of the stand-out players at the World Cup and he led Fiji from the front in all their matches.
"He's a very experienced international who adds significant strength in depth to the squad and he is a huge asset to the club.
"He has an important role not only to play the rest of this season, but also in the years ahead."
Rauluni is currently at home in Australia to be with his wife Maile and their newborn daughter Isabella.