Back-rower Tom Pearson has become the third former London Irish player signed by Northampton Saints since the club's financial collapse.
Pearson's Saints deal comes after he was named in England's 28-man pre-World Cup training squad on Monday.
He said he would "treasure" his time with Irish, where he scored 10 tries in 39 games over two years.
London Irish went into administration last week, having already been suspended from the Premiership.
Pearson, who won London Irish's young player of the year award in his first season, was among the Exiles' most in-demand players.
He was named the Premiership's breakthrough player of the season and the Rugby Players' Association's (RPA) young player of the season after helping the Exiles to a fifth-placed top-flight finish in 2022-23.
"I'll always be incredibly grateful to London Irish and treasure my time there," Pearson said.
"They gave me a shot two years ago coming out of university, when I was an unknown and probably a bit of a risk.
"It's tragic to not know the true potential of that team we had, but I want to thank all the players, staff and the fans for everything they gave me over the last two years and for helping me create lifelong memories."
Saints director of rugby Phil Dowson said Pearson has the physicality and movement to "make significant impacts all over the park".
"Tom is an incredible athlete, and is just going to get better the longer he is in the professional environment," he told the club website.