We present the results of an ultrahigh vacuum, low-temperature scanning tunneling microscopy investigation of cobalt clusters adsorbed on the surface of single-crystal molybdenum disulfide. This system is of interest as a model for understanding industrial hydrotreating catalysts. We observe small metal clusters that bind on the MoS2 basal plane, and determine from atomically resolved images that these clusters are composed of cobalt atoms bound exclusively atop surface sulfur atoms. Small clusters are also observed nucleated near surface defects. Larger cobalt clusters are observed primarily bound at MoS2 surface defects, and are additionally observed near and on surface steps. Cobalt clusters on the MoS2 surface are extremely labile and change their size and shape on the time scale of imaging. Individual atoms are observed to adjoin to and detach from clusters, possibly under the influence of the probe tip; under appropriate tunneling conditions, the tip will sweep all clusters from a scanned area. The low barrier to diffusion for adsorbed cobalt on MoS2 is in accord with earlier measurements of nickel on MoS2, and supports conclusions drawn about the action of promoter species in hydroprocessing catalysis.