  We present high-speed optical photometry of the soft gamma repeater SGR
0501+4516, obtained with ULTRACAM on two consecutive nights approximately 4
months after the source was discovered via its gamma-ray bursts. We detect SGR
0501+4516 at a magnitude of i' = 24.4+/-0.1. We present the first measurement
of optical pulsations from an SGR, deriving a period of 5.7622+/-0.0003 s, in
excellent agreement with the X-ray spin period of the neutron star. We compare
the morphologies of the optical pulse profile with the X-ray and infrared pulse
profiles; we find that the optical, infrared and harder X-rays share similar
double-peaked morphologies, but the softer X-rays exhibit only a single-peaked
morphology, indicative of a different origin. The optical pulsations appear to
be in phase with the X-ray pulsations and exhibit a root-mean-square pulsed
fraction of 52+/-7%, approximately a factor of two greater than in the X-rays.
Our results find a natural explanation within the context of the magnetar model
for SGRs.
