<p>Once abundant across all tropical and subtropical ocean basins, oceanic whitetip sharks (<i>Carcharhinus longimanus</i>) have experienced major population declines, resulting in the species now being listed as “Critically Endangered” throughout its range. The major threat to this species is overexploitation, driven in large part by retention of oceanic whitetip bycatch in tuna fisheries because of the value of derivative products such as dried fins in international trade. Retaining, landing, storing, selling and transshipping of whole or any part of oceanic whitetip sharks is now prohibited in fisheries jurisdictions governed by all major tuna Regional Fisheries Management Organizations (tRFMOs). In addition, since late 2014 the species has been listed on Appendix II of the Convention on International Trade of Endangered Species of Flora and Fauna (CITES), which means any international trade must be accompanied by an export permit certifying that the ‘take’ is not a threat to the species survival in the wild. Here, we present the first assessment of the relative contributions of Atlantic and Indo-Pacific oceanic whitetip shark populations to two major dried shark fin markets in China sampled from 2014 to 2017. CITES reporting over this period was entirely from Indo-Pacific nations, indicating that any oceanic whitetip sharks from the Atlantic Ocean in trade were uncertified, unreported, and thus illegal. Concatenated sequences of the complete mitochondrial control region (mtCR) and partial NADH dehydrogenase subunit 4 (ND4) genes were used in a mixed stock analysis, which indicated Indo-Pacific sources for most fins (81.47%) with a substantial and unreported contribution from the Atlantic (18.53%). This provides direct evidence of illegal trade from the Atlantic and highlights the need for greater scrutiny of trade from this region to achieve species conservation.</p>