The presence of PRC security forces in the Lower Mekong region has been growing and is expected to expand even more rapidly. For example, China, Laos, Myanmar, and Thailand have been carrying out joint patrols on the Mekong River since 2012. There are also reports that a quiet deal will allow PLAN ships to regularly visit new Chinese-constructed piers at Cambodia’s Ream naval base and suspicions that expansion of the Dara Sakor airfield will enable the deployment of military aircraft. The US has expressed concerns over these developments through both unofficial and official channels. However, these developments and their implications are poorly understood and under-analysed by in the public record. This study seeks to understand the nature of PRC security involvement in the Lower Mekong and explores the impacts for regional relationships and explore include the implications for the US military access and consequences for other ASEAN states.