The doctrine of Res Arbitrata renders a prior arbitral award final before subsequent courts or arbitral tribunals and bars the parties from re-asserting matters previously adjudicated. Its effects extend to both the claim (claim preclusion) and to issues determined in the earlier proceedings (issue preclusion), with the important distinction that, for issue preclusion, identity of claims across the two disputes is not required. Accordingly, national legal systems adopt either a narrow or a broad approach to defining the scope of Res Arbitrata. Models also diverge regarding the applicable framework: in most jurisdictions, the rules governing the res judicata effect are transposed to both domestic and international arbitral awards, whereas some jurisdictions prescribe separate regimes for each. As a result of these divergences, scholars and practitioners may face significant uncertainty when attempting to anticipate the preclusive effects and scope of their awards before foreign tribunals. A survey of comparative practice further indicates that, in Iran, the scope and conditions of Res Arbitrata remain seriously ambiguous, warranting substantial reconsideration in both judicial practice and legislation.