The move had been on the cards since regional rugby was rubber-stamped with the number of overseas players likely to become an issue.
But the Zurich Premiership outfit finally confirmed last night that Fiji captain Raiwalui was on his way to Vicarage Road in the summer after four years with the Black and Ambers.
"Saracens is a club that I have always been interested in and I'm very pleased to be joining up with Wayne Shelford (Saracens coach) and the squad for next season," said Raiwalui.
"I want to get my head down, work hard and start getting the respect from the guys. I want to do well, and I want to do what I can to help the club be successful.
"With Newport I've had three years of top level European rugby, and it would be great for Saracens to make it back into the Heineken Cup, it's a great environment to play in."
Raiwalui, capped 41 times by Fiji, is regarded as natural successor to French powerhouse Benazzi who is retiring at the end of the season and is highly rated by Shelford.
He said: "Simon is a big guy and top line out jumper. He also enjoys running with the ball and knows how to use his size.
"Coupled with this, he has shown at Newport and for Fiji that he has good leadership qualities which will be of use to us here. He will add solidity and power to the pack and will be another experienced player in the lock position. I'm sure that he'll be fantastic for us."
Saracens completed their new-look second-row with the signing of Alex Codling from Harlequins.