Prewar Japanese labor policies were not only shaped by industrial mutations, the development of labor unions, and domestic political changes, but rather, were also inspired by ideas that circulated from one continent to the other through international contacts among labor unionist and social reformist. To date, there has been no detailed historical analysis that systematically draws on first-hand data of how Japan’s membership in the International Labour Organization (ILO) changed the course of its labor governance strategy in the interwar period. ILO membership compelled the Japanese governing elites to face up to important questions regarding national economic development. For example, what was the better strategy to enrich and strengthen the nation: cheap, submissive labor or collective bargaining; improving working conditions to generate higher productivity? In light of the international geopolitical context that was unfolding, what was the more realistic option: free trade by means of diplomatic cooperation or expanding the country’s economic bloc by means of territorial conquest? This article draws on the rich, hitherto unused ILO archives in Geneva, to focus on the developments in relations between the ILO and Japan from the organization’s creation in 1919 through to the country’s departure in 1938. It documents the changing and contingent reasons why the ILO was an important partner for Japan, while uncovering the pivotal role played by liberal social reformists throughout this period.