Last updated on .From the section Rugby Union
Ratuniyarawa won the Premiership Rugby Cup with Northampton in 2019 and helped them reach the Premiership semi-finals last season.
The second-row has 38 caps for his country since making his debut in 2012 and has appeared in two World Cups.
"London Irish is a fantastic club, and I am aware that I am joining at a significant time in the team's journey," he told the Exiles' website .
"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."
Ratuniyarawa began his career with North Harbour in Auckland, New Zealand, in 2012 before spending three years at Agen, in France. He moved to Saints in 2016.
The length of his contract with the Exiles has not been specified.
Irish finished eighth in the Premiership last season to qualify for the European Champions Cup, while Saints finished fourth and were beaten in the semi-finals by league winners Leicester Tigers.