Harry Broderick (51) and Tom Cheater (43) top scored for South Wilts, with Rob Pitman (28) and Ben Jack (20 not out) making useful contributions later in the innings.
Archie Pascoe returned figures of 3-51 for Sarisbury, who were in the driving seat with Tom Morton (65), Nathan Feltham (44) and Chris Sanders (45) going well.
Former Hampshire seamer Richard Logan hit 45 as Sarisbury closed in in the target, only to fall just short with 267-9, South African Rhys Willmore taking 3-51.
Tom Pearce (76) and Henry Smith (41) shared an unbroken century partnership as South Wilts II overhauld ed a Bere Regis total of 201 to win by five wickets with seven balls to spare.