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We report on the archival analysis of low-mass X-ray binary MS 1603.6+2600 observed with the Rossi X-Ray Timing Explorer. The 2.5-20 keV X-ray light curve shows a broad modulation with a period of 110.1±1.0 min. The X-ray period is consistent with the optical period. The spectrum during the low-intensity phase differs from the high-intensity spectrum by an energy-independent reduction in intensity, and not due to additional absorption. Our findings concur with an earlier suggestion by Hakala et al. that the orbital intensity modulation is caused by a partial covering of an extended emission region by a thick absorber. We also estimate the distance to MS 1603.6+2600 to be <24 kpc if the X-ray-emitting region is optically thin.