Reading comprehension has recently been reconceptualized in EFL reading instruction to foreground the importance of putting a social perspective on learning. Developed as a crucial aspect of Vygotskian sociocultural theory, activity theory views reading as a socially-mediated activity, for which the prerequisite cognitive processes are distributed among teacher, individual reader, other students, and artifacts (Cole & Engeström, 1993). Given that cooperation and division of labor are the central tenets of activity theory, this study aimed at investigating whether assessing cooperative learning had a decisive effect on the reading comprehension of Iranian EFL learners. To this end, 60 sophomores majoring in English translation at Islamic Azad University, Tehran Central Branch, were selected as the participants of the study. The reading instruction was geared to cooperative learning based on the elements of activity theory. Over the course of 12 weeks, both the process and products of cooperative reading were self-, peer-, and instructor-assessed. The findings indicated that assessing cooperative reading through the lens of activity theory had a significant effect on the participants’ reading comprehension. In addition, there was a statistically significant difference between the products of cooperative reading in predicting the participants’ reading comprehension posttest scores. Furthermore, the results showed that the participants held favorable perception toward activity theory-based cooperative assessment. The findings are hoped to shine a light on collective reading and highlight the need for more innovative constructivist approaches to EFL reading in Iran.