A Fourier decomposition of the World Ocean Atlas 1998 (WOA98) fields of climatological monthly mean salinity is used to describe the annual cycle of sea surface salinity (SSS). Global data sets of river runoff and evaporation minus precipitation (E-P) are used to put the annual cycle of SSS in the context of the global climate system. Most of the world ocean is shown to have an annual cycle of less than 0.3 on the practical salinity scale (PSS). Areas with an annual cycle larger than 0.3 include the northern Indian Ocean, the tropical Pacific and Atlantic, and the northern North Atlantic.