Heerenveen went ahead on a 2nd-minute own goal before Bradley opened his account in the 7th minute by slamming home a rebound.
FC Groningen scored twice to level the game by the 14th minute, but in the 22nd minute, Bradley deftly redirected a low cross from Danijel Pranjic from 10 yards out to make it 3-2.
Bradley again finished a rebound to complete the hat trick in the 41st minute.
The American is now Heerenveen's second leading scorer, behind 10-goal Brazilian Afonso Alves.
Extended highlights (4:20 minutes)
Bradley postgame interview in Dutch