We present the optomechanical design of a polarimeter to be used with the SCUBA-2 camera at the James-ClerkMaxwell Telescope. The polarimeter, built to study polarized sub-millimeter radiations, has a clear aperture of 269 mm and is composed of three optical elements: a calibration polarizer, a half wave plate rotating at a speed of up to5 Hz, and an analyzer polarizer. All three elements can be placed in and out of the beam, depending on the telescope's observation mode.