The Pilgrims' long-serving club captain is leaving The Jakemans Stadium after agreeing the mutual termination of his contract.
Jelleyman, who enjoyed two loan spells (in 1998-1999 and 2004-2005) with United before a permanent move in 2010, made a total of 122 appearances for the club.
Reflecting on his captain's departure, United boss Graham Drury told bufc.co.uk: "I wish Gareth all the best for the future. He has been a top player for Boston United.
"His partner has had a baby in the last few hours and we wish them all well."
The player, who became a father again on Friday morning, moves on with the best wishes of everybody at Boston United.