The Southern Ocean has the lowest densities of floating macroplastic litter in the world. It was thought that the region was relatively free of microplastic contamination. However, recent studies and citizen science projects have reported microplastics in deep-sea and shallow sediments and surface waters. Microplastics have been shown, in both laboratory experiments and field-based studies elsewhere in the world, to negatively impact a range of marine species including pelagic and benthic organisms. After reviewing available information on microplastics (including macroplastics as a source of microplastics) in the Southern Ocean, we present estimated microplastic concentrations, and identify potential sources and routes of transmission into the region. Estimates suggest that the amounts of microplastic pollution released into the region from ships and scientific research stations are likely to be negligible at the scale of the Southern Ocean, but may be significant on a local scale. Furthermore, predictions of microplastic concentrations from local sources are several orders of magnitude lower than levels reported in published sampling surveys. Sea surface transfer from lower latitudes is a likely contributor to Southern Ocean plastic concentrations.