Leach has 39 Test caps for England, taking 142 wickets at an average of 34.07 but has not played for his country since the Test series defeat by Pakistan in October 2024.
Last month he was overlooked for the Ashes squad this winter in Australia.
Taunton-born Leach has overcome a number of injury setbacks that have disrupted his playing time and led to a lot of what he described as “soul searching”.
He missed the Ashes in 2023 due to a stress fracture in his back and then suffered a knee injury on his Test match return that required surgery and kept him out of playing for Somerset for a year.
“Jack epitomises everything that it means to play for this great club,” said head coach Jason Kerr.
“His performances on the field are outstanding, and his work ethic is second to none.
“Jack has a genuine passion for the club and gives nothing less than 100% every time he walks out onto the field or comes in for training. His commitment and drive set the standard for others to follow.”
