The relevance of lex mercatoria – the “law merchant” in an increasingly globalized commercial environment has resurfaced as one of the vital sources of the regulation and resolution of transnational commercial disputes. Traditionally rooted in medieval trade practices and customary rules developed by merchants themselves, lex mercatoria has evolved into a modern body of transnational commercial norms that operate independently of any single national legal system. This paper explores the nature, scope, and functional role of lex mercatoria in contemporary International Commercial Law, with a focus on its application in contracts, dispute resolution, and the harmonization of legal standards across jurisdictions. Modern lex mercatoria encompasses an array of non-state legal sources, including international commercial customs, general principles recognized by commercial actors, arbitral case law, and soft law instruments. Influence on UNIDROIT (International Institute for the Unification of Private Law) Principles of International Commercial Contracts and the Principles of European Contract Law is very visible example. It operates in parallel with, and often in support of, national and international legal systems, providing neutral, flexible, and business-friendly rules that facilitate cross-border commerce. Particularly in situations where parties opt for arbitration and choose not to subject their contracts to any specific national law, lex mercatoria serves as an autonomous legal framework that can fill legal gaps and reflect international commercial expectations. This research critically examines the legitimacy and utility of lex mercatoria in shaping modern International Commercial Law. While it offers advantages such as neutrality, uniformity, and adaptability, concerns remain regarding its vagueness, lack of formal codification, and potential for unpredictability. The paper evaluates how arbitral tribunals and international commercial courts have engaged with lex mercatoria, either as governing law or as a supplementary tool, and considers whether its use promotes legal certainty or introduces ambiguity into commercial relations. The paper argues that lex mercatoria should not be viewed as a replacement for domestic or international law, but rather as a complementary source that enhances the efficiency, coherence, and legitimacy of international commercial law. Its growing influence reflects the needs of global commerce for flexible, party-driven legal solutions that transcend traditional legal boundaries. This research contributes to the existing body of scholarship on lex mercatoria by re-evaluating its role in the contemporary framework of International Commercial Law, rather than limiting the analysis to arbitration or contract law contexts alone. While much of the academic focus has traditionally centered on lex mercatoria as a set of principles used primarily in arbitral tribunals, this paper broadens the perspective to examine its practical and normative influence on modern commercial practices, regulatory interactions, and legal harmonization efforts. This paper contributes by clarifying its normative function and assessing how courts and tribunals treat it—not just as a supplementary source, but sometimes as a self-standing framework when parties explicitly or implicitly choose it.