This study was conducted to investigate the effect of using online discussion forums (ODFs) on students' learning, particularly on their achievement. In order to achieve this, a quasi-experimental design was implemented during one academic semester at one of the leading universities in Saudi Arabia. The sample of this study involved undergraduate students (N = 138) divided into two groups: the experimental group involved 67 students and the control group involved 71 students. The findings indicate that using ODFs is likely to lead students to gain a better achievement. In addition, statistical analyses reveal significant and positive relationships between student participation in ODFs and their final course mark, but no significant relationships between their participation in ODFs and grade point average. The social interaction and the collaborative nature in ODFs environments as well as the active learning in blended learning courses were likely to be the possible reasons for the increased achievement when students utilise ODFs to enhance traditional learning. However, contextual dimensions need to be given a great deal of attention in order to find satisfactory results.