<p>This article develops the concept of <i>gangster diplomacy</i> as a practice-based analytical category for understanding contemporary transformations in international order. Situated at the intersection of practice theory and the English School, it conceptualizes diplomacy not merely as an instrument of statecraft, but as a socially embedded institution constituted through historically sedimented norms, tacit knowledge, and routinized performances. The article argues that the highly personalized and coercive diplomatic practices associated with Donald Trump represent not simply stylistic deviation, but a disruption of the normative grammar that has traditionally underpinned U.S. diplomatic conduct. By foregrounding the relational and performative dimensions of diplomacy, the concept of gangster diplomacy captures how micro-level practices of intimidation, transactional bargaining, and executive centralization can generate macro-level systemic effects. When enacted by a structurally privileged power, such practices erode shared expectations, destabilize institutional constraints, and weaken the order-producing functions of diplomacy. The article thus interprets gangster diplomacy as an indicator of a broader shift along the continuum between international society and international system. In doing so, it offers a framework for analyzing how deviations in diplomatic practice accumulate into structural transformations of global order.</p>