This paper aims to identify the lingua franca at the border of Malaysia-Thailand via thedomain concept introduced by Fishman (1972). This concept proposes several domains such as family, business, neighbourhood, friendship, work and religion, which can be used as the basis for studies on language choice in multilingual communities such as the Malaysia-Thailand community. This study focuses on business, family and tourism domains because the Malaysia-Thailand border, i.e. Rantau Panjang and Golok are popular shopping destinations. Data for the research were collected through questionnaires distributed to respondents in the Malaysia-Thailand border. Findings of the research show that respondents from Malaysia use different languages when they communicate with those from Thailand. The respondents in the 20-40 age range are more inclined to choose Kelantanese Malay dialect, Thai language or Chinese. However, older respondents (above 40 years) choose only Kelantanese Malay dialect. These results show that language choice at the Malaysia-Thailand border is influenced by the age of the respondents. The findings also show that respondents from Malaysia and Thailand are more inclined to choose Kelantanese Malay dialect in family domain. This shows that Kelantanese Malay dialect functions as the lingua franca at the Malaysia-Thailand border as opposed to Thai language. In other words, the Thai respondents have assimilated with the Malay community at the Malaysia-Thailand border.