Differentiated Instruction (DI), as a single instructional technique that focuses on the challenges of meeting varied needs and qualities of learners in inclusive educational contexts, has recently attracted great attention in the field of education. The present study aims to explore the effects of differentiated instruction (DI) on foreign language achievement (FLA), foreign language motivation (FLM), and learner autonomy (LA) of English learners at a state school in Istanbul, Turkey. Additionally, the study attempts to explore how students and their teacher perceive the use of DI in English classrooms as well. The participants were 24 students and one teacher enrolled in the 8th grade (secondary level) English program at a state school in Istanbul, Turkey. The data were collected quantitatively using the Foreign Language Motivation Questionnaire, pre-and post-achievement tests, and the Learner Autonomy Scale. To complement the quantitative data, qualitative data were gathered from student interviews and teacher reflective journals. The findings demonstrated that incorporating DI enhanced the participants' overall FLA as well as FLM and LA. Both students and their teacher perceived DI to be effective and useful while studying and teaching English. Based on the obtained findings, the study provides suggestions and pedagogical implications about incorporating DI in secondary-level English classrooms.