The seasonal cycle of planetary boundary layer (PBL) depth is examined globally using observations from the Constellation Observing System for the Meteorology, Ionosphere, and Climate (COSMIC) satellite mission. COSMIC uses GPS radio occultation to derive the vertical profile of refractivity at high vertical resolution (~100 m). Here, we apply an algorithm to determine PBL top height and thus PBL depth from the maximum vertical gradient of refractivity. PBL top detection is sensitive to hydrolapses at nonpolar latitudes but to both hydrolapses and temperature jumps in polar regions. The PBL depths and their seasonal cycles compare favorably with selected radiosonde-derived estimates at tropical, midlatitude, and Antarctic sites, adding confidence that COSMIC can effectively provide estimates of seasonal cycles globally. PBL depth over extratropical land regions peaks during summer consistent with weak static stability and strong surface sensible heating. The subtropics and tropics exhibit a markedly different cycle that largely follows the seasonal march of the Intertropical Convergence Zone with the deepest PBLs associated with dry phases, again suggestive that surface sensible heating deepens the PBL and that wet periods exhibit shallower PBLs. Marine PBL depth has a somewhat similar seasonal march to that over continents but is weaker in amplitude and is shifted poleward. The maximum seasonal amplitude over oceans occurs over the Arctic. Over subtropical/tropical oceans there is seasonal asymmetry about the equator, with winter maxima in the Northern Hemisphere but fall maxima in the south. The seasonal march of PBL depth is largely modulated by the seasonal cycle of static stability in the extratropics and by the monsoon circulations at tropical and subtropical latitudes.