Meaningful participation in science and engineering practices requires that students make their thinking visible to others and build on one another's ideas. But sharing ideas with others in small groups and classrooms carries social risk, particularly for students from nondominant groups and communities. In this paper, we explore how students' perceptions of classrooms shape their contributions to classroom knowledge building in science across a wide range of classrooms. We examine the claim that when students feel a sense of belonging in class, they contribute more and perceive their ideas to be more influential in knowledge building. Data comes from classroom exit tickets (n = 10,194) administered in 146 classrooms as part of a 10-state field test of a new middle-school science curriculum, OpenSciEd, which were analyzed using mixed effects models. We found that students' sense of belonging predicted the degree to which they contributed ideas out loud in class (Odds ratio = 1.57) as well as the degree to which they perceived their contributions as influencing others (Odds ratio = 1.53). These relationships were particularly strong for students who reported a lower a sense of belonging. We also found significant differences by both race and gender in whether students said they contributed and believed their ideas influenced those of others. These findings suggest that a learner's sense of belonging in class and willingness to contribute may be mutually reinforcing, highlighting the need to promote content-specific strategies to foster belonging in ways that support collaborative knowledge building.