Krill habitats in the Southern Ocean are impacted by changing climate conditions, reduced sea ice and rising temperatures. These changes, in turn, affect krill occurrence, physiology and behaviour, which could have ecosystem impacts. In this Review, we examine climate change impacts on Antarctic krill and the potential implications for the Southern Ocean ecosystem. Since the 1970s, there have been apparent reductions in adult population density and the occurrence of very dense swarms in the northern Southwest Atlantic. These changes were associated with latitudinal and longitudinal rearrangement of population distribution — including a poleward contraction in the Southwest Atlantic — and were likely driven by ocean warming, sea-ice reductions and changes in the quality of larval habitats. As swarms are targeted by fishers and predators, this contraction could increase fishery–predator interactions, potentially exacerbating risk to already declining penguin populations and recovering whale populations. These risks require urgent mitigation measures to be developed. A circumpolar monitoring network using emerging technologies is needed to augment existing surveys and better record the shifts in krill distribution.