Phil Dowson will join Worcester Warriors when his Saints contract ends this summer.
The 33-year-old flanker had been keen to remain at Franklin’s Gardens and was offered a one-year extension, but the lure of two years at Worcester proved too strong.
Dowson agreed the deal with the Championship table-toppers last week, but wanted to wait until after Saints’ game against Wasps to inform his team-mates of the move.
He did so at the team briefing on Monday morning, after skippering Saints to a 29-6 victory on Saturday.
Dowson has been a key figure at the Gardens since making the move from Newcastle in the summer of 2009.
He has made 179 appearances for Saints, captaining the club to LV= Cup glory in 2010 and playing a major role in last Aviva Premiership and Amlin Challenge Cup triumphs.
The void he will leave at Saints will be filled by Jamie Gibson, who has agreed to join Saints from Leicester Tigers at the end of the season.