This study examined the impact of cohesive devices in English textbooks on Iranian high school students' reading comprehension. 64 students were divided into experimental and control groups. The experimental group read passages from a textbook that highlighted cohesive devices, while the control group practiced past exam questions. Both groups took a pre-test and post-test of reading comprehension. Results of t-tests and effect size calculations showed that the experimental group performed significantly better on the post-test, indicating that exposure to cohesive devices in texts helped improve their reading comprehension. A correlation also suggested that students with higher English proficiency benefited more from the cohesive devices. The findings suggest explicit instruction of cohesive devices could aid EFL reading comprehension.