The 35-year-old has been a key figure at the Gardens in recent years, having made the move from French side Agen back in 2016.
He racked up 130 appearances in his six seasons at Saints, helping the club to finish in the top four of the Premiership on two occasions.
Ratuniyarawa was named breakthrough player of the season back in 2017/18 and the Fiji ace has gone from strength to strength ever since.
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But he will now be making the move to Irish.
“London Irish is a fantastic club, and I am aware that I am joining at a significant time in the team’s journey," Ratuniyarawa said.
“I am really excited about taking this next step in my rugby career, and having played in Brentford already this year, I now can’t wait to pull on the green jersey and play in front of the London Irish fans.”
Irish boss Declan Kidney is looking forward to welcoming Ratuniyarawa to the Brentford Community Stadium.
“Api is a very experienced player who we identified as a good fit for the team, and he will be an important member of the squad at London Irish next year," Kidney said.
“He has many notable attributes we hope will aid us in furthering our results on the pitch and assist in the development of the younger members of this club.”