Thai government schools have become multicultural due to an increasing number of migrant students whose parents are from neighboring ASEAN countries. Although published English language teaching (ELT) materials have been researched extensively, supplementary materials that are integrated as additional resources for teachers and students such as workbooks or activity books used in Thai government schools are under-researched. This study investigated the cultural diversity content represented in six ELT activity books used in a primary school located in Samut Sakhon province in Thailand. Cortazzi and Jin's (1999) cultural concept was employed as a framework to analyze layers of cultural diversity represented in the selected ELT activity books. Drawing upon content analysis, the findings reveal six cultural sources ranked in the following order: international cultures (40%), target cultures (23%), global cultures (17%), Asian regional cultures (16%), unidentified sources (4%), source cultures (0%), and ASEAN regional cultures (0%). Diverse international cultures are frequently incorporated into these activity books; however, source cultures referring to students' sociocultural backgrounds and ASEAN regional cultures are under-represented. This study proposes pedagogical implications for teachers who use local ELT activity books provided by the government, with regard to adapting and designing culturally appropriate lessons and activities for students in multicultural primary schools in Thailand in order to meet UNESCO SDG 4 (Quality Education).