Supporting productive peer-to-peer interaction is a central challenge in online courses. Although cooperative learning is a well-established method for eliciting productive interaction (Johnson & Johnson, 1989), online modes of cooperative learning have provided mixed results. To explore the factors underlying these mixed results, we used a quasi-experimental design to examine the effects of synchrony on students' sense of cooperation, belonging, affect, and cognitive processes in online small-group, discussion-based cooperative learning. Fifty-two undergraduate students were assigned to synchronous and asynchronous interaction conditions in an online course. The findings support prior research that asynchronous communication interferes with the relationship between cooperative goals and the outcomes of cooperation. The results extend the literature by indicating that synchrony positively affects students' perceptions of belonging, positive affect, and cognitive processes. Results inform theory and practice by showing that asynchronous cooperative learning may not work as designed due to students' lack of perceptions of interdependence.