An academy product, the back rower made 128 appearances for Northampton across eight seasons and was appointed club captain in 2021.
Capped 25 times for his country, Ludlam featured at the 2019 and 2023 World Cups but will be unavailable for selection if he moves to the Top 14.
“Ever since I started playing rugby as an eight-year-old, I dreamt of being a Saint,” Ludlam, who missed out on selection for Steve Borthwick’s Six Nations squad, said. “While the journey hasn’t always been smooth sailing, our supporters have made every moment special for me – both the good and the bad.
“I feel extremely fortunate to have played with some great players and even better blokes, to have been coached day in, day out by some of the best coaches in the game. At the end of the season, I say goodbye as a player to start a new adventure, but I will always be a Saints man.”
Phil Dowson, the club’s director of rugby, said the club were “disappointed” to lose a figure of such stature.
“He’s the epitome of what we try to do with players at the club,” Dowson said. “He came in as a youngster through the academy system, got straight into work training with the first team, and has transitioned into a key player for us as well as being the captain of the club – and he’s done an exceptional job leading the group, driving the energy and standards here.”
Northampton could also lose fellow stalwart back rower Courtney Lawes at the end of the season.
Lawes is understood to be considering an offer from the club that could keep him at Saints but is also the subject of interest from a number of French clubs.
Brive are thought to be leading the race for the 34-year-old, who concluded his England career after the Rugby World Cup in the autumn.