Distance learning and online learning have become the new educational paradigm. Based on synchronicity, online learning environments can be classified into synchronous and asynchronous online learning. However, previous evidence demonstrating the effects of these two online learning modalities on students' academic achievement has been contradictory. The current meta-analysis study pooled the observed effect sizes from previous research and addressed the following two research questions: (1) Which online learning format (synchronous or asynchronous) generates stronger learning effects? (2) Do the effects vary by the disciplinary field and educational level in which the learning is carried out? A systematic search of studies published between 2002 and 2022 was conducted. A total of 14 studies with 1,098 participants for the synchronous learning condition and 804 participants for the asynchronous learning condition met the study selection criteria. The results showed that asynchronous learning was more effective in promoting student knowledge than synchronous online learning, but the effect was trivial in size. The overall effect size was largely invariant across educational levels and disciplines. The implications of this study are also discussed.