Energetic protons (2.8–78 keV) and water group ions (8.8–78 keV) observed in Saturn's magnetosphere using the Magnetospheric Imaging Instrument/Charge‐Energy‐Mass Spectrometer instrument from 2005 through 2012 were subjected to a Lomb periodogram analysis with the period window extending from 0.5 to 50 days, and the data constrained to the spatial region between 10 RS (1 RS = 60,268 km) and the magnetopause. Both the protons and water group ions exhibited solar periodicity at ~26 days and harmonics thereof. For all ions, the 26 day periodicities were strong on the dayside and duskside, weak at dusk, and virtually nonexistent on the nightside. The solar periodicity was evident during both the first and second halves of the 7 year time span, but strongest in the first half from January 2004 to July 2008. Some of the ion spectral peaks, especially for the water group ions, can be associated with orbital motion of the spacecraft. The 26 day periodicities are likely caused by corotating interaction regions in the solar wind that periodically sweep past Saturn, regularly compressing its dayside magnetosphere, but not its nightside.