Jamie Gibson has opted to swap Leicester for Saints with the flanker agreeing to move to Franklin’s Gardens this summer.
The 24-year-old will bolster a back row that already includes England duo Calum Clark and Tom Wood.
Gibson has made 42 appearances for Leicester since joining the club from London Irish in the summer of 2013.
And Saints boss Jim Mallinder believes his new recruit has a big future at the Gardens.
“Jamie has shown repeatedly at London Irish, Leicester and in England representative teams that he is a player with a big future, and we’re delighted that he has put pen to paper to join the Saints,” Mallinder said.
“Back row is an important part of the team, and while we have some outstanding talent in that area we are confident that Jamie can supplement the players that we already have.
“However even though he has played a lot of games he is still relatively early in his career, and we believe that he has the potential to improve even further.”