Destination states of irregular migration aim to prevent arrivals by controlling their borders outside their territory, specifically on the high seas. This practice may best be described as the de-territorialization of border control at sea. The de-territorialization impacts the applicable legal framework, in particular the safeguards to which individuals submitted to the control activities are entitled. This article posits that the principle of non-refoulement is a fundamental yardstick for the de-territorialization of border control and applies wherever competent state authorities perform border control measures. The argument develops in four steps. After outlining the content of the principle of non-refoulement, this article defines maritime borders and elucidates their functional nature. It then outlines how the principle of non-refoulement applies at sea and translates into a ‘principle of non-rejection at the maritime frontier’. The article finally highlights the principle's legal and practical consequences in the context of de-territorialized border control.