Improving the understanding and treatment of mental health concerns, including depression and anxiety, are significant priorities for autistic adults. While several theories have been proposed to explain the high prevalence of internalizing symptoms in autistic populations, little longitudinal research has been done to investigate potential causal mechanisms. Additional research is needed to explore how proposed contributors to depression from general population research predict and/or moderate the development of internalizing symptoms in autistic individuals. In this study, we investigated the relation of one established risk factor, repetitive negative thinking (RNT), to internalizing symptoms over the course of college students' first semester, additionally examining whether this association is moderated by a measure of autistic traits. Students were recruited from 4 northeastern U.S. universities: 144 participating students included 97 nonautistic students and 47 participants who either reported a formal autism diagnosis (n = 15) or endorsed a history of self and/or others thinking that they may be autistic (n = 32). Participants completed a baseline survey battery within their first 2 weeks of starting college, a brief biweekly survey throughout their first semester (up to 24 times across 12 weeks), and an endpoint packet. Elevated trait-like RNT at baseline was prospectively associated with biweekly ratings of depression and anxiety symptoms across the semester. In addition, greater RNT was synchronously related to elevated sadness, anhedonia, and anxiety throughout the semester. Contrary to hypotheses, a shorter term predictive relationship between RNT at one timepoint and mood symptoms at the next was largely unsupported. While these patterns were observed across neurotypes, students with higher self-reported autistic traits were more likely to experience RNT, as well as depressive and anxiety symptoms. These preliminary findings highlight RNT as a specific mechanism that may be a useful prevention and/or intervention target toward reducing the elevated depression and anxiety rates in the autistic community.