Malaysia is a federal constitutional monarchy that shares a land border and maritime boundary with Brunei. About 60% of its population are Muslims, mostly Sunni of Malay ethnicity.
Indonesia is a unitary presidential republic that occupies most of the island of Borneo along with Brunei. It is the most populous Muslim-majority country in the world, with about 87% of its population adhering to Islam, mainly Sunni.
Singapore is a sovereign island city-state that lies south of the Malay Peninsula, across the Strait of Johor from Brunei. It has a diverse religious landscape, with about 15% of its population being Muslims, mainly Malay and Indian.
Thailand is a constitutional monarchy that is situated on the Indochinese Peninsula, southeast of Brunei. It is a predominantly Buddhist country, but has a sizable Muslim minority of about 5%, mostly concentrated in the southern provinces.
Cambodia is a constitutional monarchy that is located in the southern part of the Indochinese Peninsula, east of Thailand and south of Laos. It is a mainly Buddhist country, but has a Muslim minority of about 2%, mostly Cham and Malay.