The work examines the interaction between the dispute settlement mechanisms established under the UNCLOS and the WTO Agreement, while exploring the challenges that multifaceted disputes straddling different treaty regimes pose to international courts and tribunals of limited jurisdiction such as the WTO DSB and UNCLOS courts and tribunals. It addresses these challenges through the lens of the WTO treaty and the UNCLOS, while providing answers to the following questions: to what extent the mentioned specialized adjudicatory bodies can refer to other rules of international law, especially treaty rules, given their limited jurisdiction; what the implications of the pronouncements of the UNCLOS courts and tribunals are with respect to the WTO DSB and vice versa; how should they approach multifaceted disputes involving both WTO law and law of the sea issues; what rules govern their interaction. The work examines and systematizes the latter rules, while particularly focusing on res judicata. Concerning res judicata, it tackles the questions what the status and meaning of res judicata is and to what types of preclusive pleas it can give rise in international law; whether it can operate as an inter-systemic rule. The work proposes solutions in case a multifaceted dispute allegedly involving different treaties and different branches of international law is submitted for resolution before different dispute settlement fora of limited jurisdiction and in doing so it contributes to the discussion on international procedural law and interaction of treaties and dispute settlement mechanisms.