A single/dual-wavelength switchable bidirectional Q-switched fiber laser using a bidirectional fiber polarizer is demonstrated. A 45° tilted fiber grating is used as a bidirectional fiber polarizer to induce a bidirectional intracavity birefringence filter in both clockwise (CW) and counter-clockwise (CCW) directions. A carbon nanotube saturable absorber is employed to produce Q-switched pulses. Through adjusting polarization states, switchable single/dual-wavelength lasing at 1551 and 1560 nm can be achieved in both CW and CCW directions. To the best of our knowledge, this is the first demonstration of a wavelength switchable bidirectional passively Q-switched fiber laser.