The 22-year-old achieved match figures of 5-78 on his first-class debut for the club in 2013, but injuries have restricted his appearances since then.
"All Tom needs is an injury-free period," managing director of cricket Angus Fraser said.
"When he gets this, his career will move forward very quickly."
Helm has been included in the England Lions squad to tour Sri Lanka in February and March.
Middlesex pace bowler Tom Helm has signed a new contract with the club, keeping him at Lord's until the end of the 2019 season.