The growing defence cooperation between Japan and Sri Lanka marks a notable strategic shift in the evolving security landscape of the Indo-Pacific region. Traditionally, Japan-Sri Lanka relations were strengthened by economic aspects, and partnerships were related to the economic development framework. This chapter explores the intricate aspects of this cooperation by investigating the motivations and tangible outcomes of their increasing cooperation. Positioned strategically in the Indian Ocean, Sri Lanka is of vital maritime significance, and Japan’s “Free and Open Indo-Pacific” initiative requires strong regional partnerships. This study examines the alignment of these interests, emphasising how mutual concerns about maritime security, freedom of navigation, and regional stability strengthen defence ties. The key elements of this collaboration include joint maritime drills, capacity-building efforts to boost Sri Lanka’s naval strength, and discussions on maritime domain awareness. Additionally, the analysis evaluates the geopolitical backdrop, considering the role of other regional players and the implications of this partnership for broader Indo-Pacific dynamics. It also addresses the potential challenges and future directions of this defence relationship, considering factors such as economic limitations and the delicate balance of regional power. Ultimately, this chapter aims to shed light on the strategic importance of Japan-Sri Lanka defence cooperation in maintaining a stable and secure Indo-Pacific region.