In the contemporary era, non-governmental actors have increasingly entered the arena of safeguarding national interests in their comprehensive sense, alongside traditional state actors. In this context, the role of parliaments—through multilateral parliamentary diplomacy as a form of complementary diplomacy—has become increasingly consistent and enduring, particularly over the past few decades. This development has drawn the attention of researchers in various countries. In this regard, the Islamic Republic of Iran and the Russian Federation are among the states that have taken the initiative in this field by establishing regional and trans-regional parliamentary assemblies. Given this fact, a key question arises: what role can multilateral parliamentary diplomacy play in advancing relations between the two countries? To address this question, the study employs available library and internet sources, applying an exploratory and comparative reading of texts within the framework of the rational choice institutionalism approach. It is assumed that inter-parliamentary relations and cooperation between the two countries—considering the functions of parliaments—can serve as a facilitating factor in developing bilateral and multilateral relations aimed at securing their mutual interests. Accordingly, this article begins with an introduction outlining the importance of multilateral parliamentary diplomacy, its historical context, research background, and theoretical foundations. It then examines the evolution of multilateral parliamentary diplomacy in both countries, followed by an analysis of the capacities for inter-parliamentary cooperation in strengthening bilateral relations. Finally, the article discusses the prospects for further development of multilateral parliamentary diplomacy between Iran and Russia.