We report on optical imaging of the X-ray binary SAX J1808.4-3658 with the 8m Gemini South Telescope. The binary, containing an accretion-powered millisecond pulsar, appears to have a large periodic modulation in its quiescent optical emission. In order to clarify the origin of this modulation, we obtained three time-resolved r'-band light curves (LCs) of the source over five days. The LCs can be described by a sinusoid, and the long time-span between them allows us to determine optical period P=7251.9s and phase 0.671 at MJD 54599.0 (TDB; phase 0.0 corresponds to the ascending node of the pulsar orbit), with uncertainties of 2.8s and 0.008 (90% confidence), respectively. This periodicity is highly consistent with the X-ray orbital ephemeris.