The 25-year-old has linked up with the Pilgrims after leaving champions Barnet.
His title success with the Bees followed Grimsby Town’s run to glory through the National League play-offs in 2022.
The right-sided defender, who possesses a long throw, will link up with Graham Coughlan’s full-time outfit when they report back for pre-season training at the start of July.
Cropper’s arrival follows hot-on-the-heels of the acquisition of goalkeeper Rhys Lovett - with more signings, new and existing, set to unfold over the coming days.