Examination of Cassini magnetic field and plasma data in the outer boundary regions of Saturn's magnetosphere shows that magnetopause oscillations at the planetary period commonly occur, in phase with plasma pressure variations inside the magnetosphere. The peak-to-trough amplitude of the boundary oscillations mapped to the planet-Sun line is estimated to be typically ~2 RS, corresponding to a ~10% change in the boundary radius. The change in internal pressure required to produce such motions is estimated to be ~40% of the background values. A qualitative physical picture is proposed in which a compressive wave propagates outward through the sub-corotating outer magnetospheric plasma, originating from a corotating source in the nearer-planet region.