Students often experience social and psychological barriers to success in General Chemistry, which is a key gateway to many students' science pathways. Learning assistants (LAs) have the potential to reduce these barriers and to strengthen students' sense of belonging in General Chemistry and STEM more broadly. Here, we used a 17-item Likert scale to determine whether incorporating LAs into General Chemistry I and II enhances students' sense of belonging in these courses. The incorporation of LAs into General Chemistry I had a significant positive effect and a medium to large effect size for students in all student groups examined: women and men; students in both racially and ethnically underrepresented and well-represented groups; first- and continuing-generation students. In General Chemistry II, similar results were observed for women and men; students in well-represented racial and ethnic groups; continuing-generation students. Further, we asked students to reflect on the impact that working with LAs had on their sense of belonging in STEM and confidence in talking about science. Sixty percent of students indicated that working with LAs had a positive impact on their STEM belonging, with five themes describing LA impacts: reducing isolation, serving as inspirational role models, providing mentoring, increasing opportunities for engagement and confidence building, and serving as accessible and approachable sources of support. Sixty-one percent of students also indicated that working with LAs increased their confidence in talking about science, with three themes emerging: fostering an environment with a lower risk of negative judgment, providing increased opportunities for feedback, and supporting students as they practiced their growing skills. Together, these results indicate that LAs can be an important means to reduce social and psychological barriers for students in gateway science courses, increasing their sense that they belong to the class and STEM more broadly.