Saints confirmed that their club captain, who was offered a new contract, would depart following eight seasons in the senior squad at Franklin's Gardens.
Ludlam, who has 25 England caps, has been linked with a move to the Top 14 in France.
If the back-row forward heads abroad he would no longer be eligible for England selection under Rugby Football Union rules.
Ludlam, who has made 121 Northampton appearances, said it had been a tough decision.
"I believe I have given everything physically and emotionally to this club and I hope our fans have seen how much it has meant to me to represent Northampton in the way I play the game," he said.
"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."
Rugby director Phil Dowson said the club were "disappointed to see a player of Lewis's ability and character move on".
"He has made a difficult decision and we have to accept and respect that, but it doesn't make it any less disappointing to see him move on, after we offered a new contract and tried to keep him at Saints," he added.
