evidence: As an archipelagic country, Indonesia has a long maritime border that raises the potential for disputes with neighbouring countries, especially regarding maritime boundaries. One of the maritime borders still in dispute is the Indonesia-Malaysia maritime border in the Sulawesi Sea, especially in the Ambalat Block area. The Indonesian Government has included the Ambalat Block as part of Indonesian territory. It has exploited the area, as it is a natural continuation of the Kalimantan continental plate and is within 200 nautical miles of the baseline. On the other hand, the Malaysian Government published a Defence White Paper in 2020, which displays a map of the Ambalat Block as part of the State of Malaysia. The boundary dispute in the waters of the Ambalat Block has led to violations of sovereignty and illegal activities, such as drug smuggling, shipping violations, IUU fishing, illegal migrant workers/trafficking and illegal trading. Therefore, this research aims to analyse Indonesia- Malaysia border security operations to improve maritime security and support national defence. The research method used in this study is qualitative. The qualitative method is used to formulate phenomena and information related to the Indonesia-Malaysia border security operations in the Ambalat Block.