Brunei Darussalam is a Malay and Islamic state that is well known for its various local cultural heritage and Adat (customs and traditions) that are steadfastly maintained until today. Adat is considered to be one of the most significant local practices that reflects the unique identity and foundation of Brunei Malay society and culture. It is part of being Calak Brunei and has been carried out and passed down from one generation to another. Adat functions as a social, political and cultural marker of Brunei Malay society. However, with exposure to and influence of Islamisation in Brunei Darussalam, there has been a decline in the performance and practice of Adat especially in Malay traditional marriage customs as some of these customs are seen to be religiously incompatible. This paper explores and understand the extent to which tension and conflict between Adat and Islam exist in practices in the marriage customs of Brunei. It will also take into account the various changes and negotiations made to accommodate Adat within Islamic practices and values.