Endeavors have been invested to account for inefficient instruction, which is frequently attributed to disregarding learner idiosyncrasies. As a matter of course, differentiated instruction has begun to receive scholarly and professional attention in the hopes of ameliorating learner outcomes through learner-aware teaching. Despite a great deal of recorded research, little is known about how differentiated instruction is prepared and operationalized. This study, conducted at a private high school in Istanbul, Turkey, investigated the impact of differentiated instruction on Turkish L2 learners' L2 achievement along with the perceptions of learners and teacher. Two intact classes were selected as control (N=14) and DI group (N=8). The control group was exposed to traditional instruction, while the DI group received differentiated instruction guided by the principles of constructivism, multiple intelligence theory (Gardner, 1993), and the differentiated instruction framework of Tomlinson (1999). Data collected by means of L2 achievement tests revealed that DI group outperformed the control group in overall L2 achievement. Learner reflective essays revealed that Turkish L2 learners found differentiated instruction as distinctive, entertaining, engaging, instructive, and interest-related, while teacher reflective journals raised the issues of time constraints, needs for learner awareness and training about differentiated instruction.