The adoption of differentiated assessment (DA) in English as a Foreign Language (EFL) classrooms in Indonesia faces considerable hurdles, largely due to teachers’ limited comprehension of the concept. This study sought to examine EFL teachers' understanding and application of DA in language classrooms, as well as to determine how their perceptions are mirrored in their practices. Utilizing a mixed-methods approach, the research combined quantitative data with qualitative insights. Data collection involved an online survey through Google Forms and subsequent phone interviews. A total of 24 secondary school English teachers participated, with six being interviewed in-depth. The quantitative data were analyzed descriptively to find the mean score (M) and the standard deviation (SD) of each item and inferentially through a paired samples t-test, while qualitative data underwent thematic analysis. The findings from the survey suggest that EFL teachers do incorporate their understanding of DA into their classroom practices. The interviews reinforced this by showing that teachers utilize DA principles through various assessment formats, such as pre-assessments, ongoing assessments, and summative assessments. Nevertheless, some inconsistencies were noted between the EFL teachers' theoretical knowledge and their practical implementation of DA. The findings of the study highlight the importance of better training and professional development for teachers, specifically in relation to DA practices, to tackle the existing discrepancies in teachers’ knowledge and practice. Given the significance of DA principles for thorough implementation, it is crucial for EFL teachers to familiarize themselves with these concepts. Moreover, since the heavy workloads of teachers pose a challenge to effective DA implementation, it is hoped that teachers will improve their time management skills to optimize the use of DA in the language classroom.