The introduction of alien fish species (AFS) into freshwater ecosystems is a major driver of biodiversity loss with important implications for African inland fisheries. This review analyses introduction pathways, establishment patterns, ecological impacts and management responses for AFS in Nigerian and Tanzanian waters. In Nigeria, aquaculture, mosquito biocontrol, ornamental fish trade and accidental releases have promoted the establishment of species such as Gambusia affinis and Poecilia reticulata in urban and disturbed systems, where they compete with native fishes and alter trophic dynamics. In Tanzania, stocking of Lates niloticus in Lake Victoria caused severe declines in endemic cichlids, restructured food webs and reshaped fisheries production and export markets. Anthropogenic disturbance, weak regulation and limited monitoring have facilitated AFS spread in both countries. The review highlights the need for stronger risk assessment, biosecurity, surveillance and regional policy harmonisation to balance food security goals with conservation of native fish biodiversity.