ABSTRACT Students across grades K-12 may communicate their mathematical understanding via writing. Mathematical writing (MW) can include journaling about prior mathematical experiences, reflecting on a problem-solving strategy, providing feedback on a peer’s strategy, and generating a mathematical proof. Although there are a wide range of MW activities, many teachers report that they do not provide MW instruction or utilize writing activities with their students. Such instructional decisions are tied to teachers’ prior experiences with MW, beliefs about MW, and self-efficacy in teaching MW. Thus, in an effort to increase MW instruction for K-12 students, this study examined pre-service teachers’ (PSTs’) prior MW experiences and the impact of an online learning module on self-efficacy in teaching, beliefs about, and instructional decisions for MW. Participants were 111 elementary, special, or dual-education PSTs at one of the two universities. Findings indicated that PSTs had minimal prior MW experiences in their K-16 schooling but believed that MW was important before the module. After the module, PSTs’ self-efficacy in teaching MW significantly increased. Further, PSTs’ saw greater benefits of MW and demonstrated more sophisticated ways to utilize MW in mathematics teaching and learning.