This article analyses the roles of the European Parliament and the US Congress in addressing regulatory interdependencies arising in the EU–US strategic partnership. It examines their international actorness as a potential remedy for the problems of democratic participation, executive dominance, and opaqueness in the shaping of transatlantic relations. It shows that legislatures significantly contribute to regulatory discrepancies and trade disputes and that the adverse consequences thereof justify more intensive ex ante cooperation between them. The analysis conducts two groups of case studies to demonstrate how the EP and Congress influence law and policy in areas of transatlantic regulatory and foreign policy divergence. The first group of case studies analyses parliamentary involvement in the making of international agreements (TTIP and ACTA). The second group of case studies inspects legislative action with extraterritorial effects (US Helms–Burton and Sarbanes–Oxley Acts). The article argues that the EP and Congress have so far frequently acted against the spirit of the strategic partnership in ways that are injurious to the interests of the other side, and discusses whether an interparliamentary early warning mechanism could reduce legislative and political frictions and increase the coherence of transatlantic lawmaking.