Online discussion forums (ODFs) offer students and teachers the opportunity to harness the endless power of the internet for educational purposes. This study investigates the impact of Edmodo as an asynchronous ODF on six of the most recurring reading comprehension skills in the TOEFL iBT test among Iranian learners. Online and offline treatments were given to 26 students in the experimental group and only offline treatments to 33 students in the control group. The statistical analysis of the data represents a large effect size (Partial η2=.234; r=.826; Partial η2=.397; r=.661) for identifying factual information, making inferences, guessing vocabulary from context, and inserting texts in the passage skills respectively, a moderate effect size (r=.363) for the understanding the rhetorical purpose of the passage, and a weak effect size (r=.156) for the identifying referential relationships between the words in the passage skills. The results tell us that Edmodo is effective in teaching reading comprehension skills by overcoming the usual time constraints and offering students autonomy in going online at the most convenient time and place. Finally, the findings are valuable for educational policymakers, curriculum designers, materials developers, language instructors, and language learners.