Refugees and other distinct migrant populations often travel together. The policy concept of “mixed migration” arose to describe this migration phenomenon. However, the term has various meanings. These can be divided into two categories: on the one hand are understandings that focus solely on the complex composition of migration flows; on the other are meanings that consider both complexity and individuals’ mixed motivations for moving. Because of this, “mixed migration” has contributed less to thinking around, and humanitarian action in relation to, migration than it might otherwise have. This article describes these diverse understandings and harnesses relevant legal principles − drawn from refugee, human rights, humanitarian, and transnational criminal law, as well as from the law of the sea − in support of one understanding of the term. It argues that international law augers in favour of an understanding focused solely on complexity, because the legal principles applicable in mixed migration situations apply regardless of individual motivations. Including such motivations within the policy concept only serves to divorce “mixed migration” from its legal underpinnings. Moreover, understanding “mixed migration” in terms of varied individual motivations for moving might fuel populist anti-immigration sentiment. A complexity-based understanding of mixed migration would enhance the concept’s utility.