Students in a fully online learning environment have limited opportunities to talk about math problem solving. While discourse is an effective pedagogical model, less is known about how it translates to online math learning. We analyzed online platform data from two fully online virtual elementary schools in the United States to address the following research questions: (a) Is participation in math discourse about reasoning and problem solving related to students' math confidence, self-efficacy toward math, and math mindset in an online learning environment? (b) Is participation in math discourse related to math performance? The study design involved school-level voluntary student participation and a retrospective analysis of online platform data. The results showed that repeated participation in discourse sessions in a course was not directly related to changes in math confidence, self-efficacy toward math, and math mindset. However, student participation in higher numbers of sessions was associated with higher final math course score and math state assessment performance, after controlling for prior performance and confidence, self-efficacy, and mindset scores. This study has important implications for the future design and implementation of online math instruction as well as the training and development of fully online math teachers.