The 28-year-old, now in his sixth season with the club, has made 77 appearances and scored 25 tries.
Northmore joined Harlequins in 2019 after graduating from Cardiff Metropolitan University and has become a key player in the midfield.
He says, “Harlequins feels like home now, and I’m thrilled to re-sign. There’s great talent in the squad, and I’m excited for what’s ahead.”