© 2019, The Author(s).This paper explores the role of transnational partnerships within a transboundary policy problem, namely illegal, unregulated, and unreported (IUU) fishing. It focuses on an understudied aspect in the partnership literature, namely ‘how and why do partnerships engage in advocacy’? The article theorizes and empirically explores the variation in strategies used by transnational partnerships to shape IUU policy development and implementation, drawing on theories from comparative politics and international relations. The paper finds that transnational partnerships often combine inside strategies with service provision, but that they rarely use outside strategies, and analyzes this variation in strategies by looking at changes in issues complexity, institutional complexity, and salience for state concerning IUU fishing policy. The paper ends by discussing the implication of these findings in relation to the previous literature on interest groups in comparative politics and on international non-governmental organizations and transnational partnerships in international relations.