The main purpose of this study is to investigate in-service teachers’ familiarization of the CEFR-aligned school-based assessment (SBA) in the Malaysian secondary ESL classroom. It also intends to explore teachers’ knowledge, understanding, and perceptions of the CEFR-aligned SBA. The study also examined the implementation of the SBA and the challenges that TESL teachers faced embracing the CEFR-aligned SBA in their ESL classroom. An exploratory mixed-method research design was employed. Data were collected by administering a survey to 108 in-service teachers, and 12 in-service teachers participated in the interview. The results show that the in-service teachers have rather a good level of familiarization with CEFR-aligned SBA and a moderate level of awareness and comprehension of the CEFR-aligned SBA. However, the in-service teachers are aware of the importance of CEFR-aligned SBA to assist students to improve their proficiency. In-service teachers exhibit a good understanding of selecting the appropriate assessment tools and methods to assess students’ learning. In-service teachers expressed their struggles and concerns regarding implementing CEFR-aligned SBA effectively, including lack of training, sourcing for good materials to teach, students'' negative attitude towards the teaching and learning process, students’ attendance, time constraint and their workload. In conclusion, the implementation of the CEFR-aligned SBA is crucial as it is a national agenda and teachers’ involvement in executing the assessment is obligatory.