People, particularly undergraduate students, who report elevated symptoms of posttraumatic stress disorder (PTSD) are at elevated risk of binge drinking. The present study used ecological momentary assessment to (a) evaluate whether PTSD severity, specifically, or psychological distress, generally, are associated with binge drinking and (b) examine the self-medication and susceptibility models of the comorbidity of PTSD with binge drinking while accounting for shared liability (i.e., the between-person association of PTSD symptom severity with binge drinking). Within a larger study of undergraduate student mental health, for 14 days, students who reported a potentially traumatic experience (N = 276) reported nightly on use of alcohol and psychoactive substances and thrice daily on current affect, internalizing symptoms, and PTSD symptoms. Daily binge drinking, per the NIAAA definition, was analyzed using multivariate mixed effects, multilevel logistic regression. Results support the self-medication model; participants were more likely to binge drink on days marked by elevated PTSD symptoms, OR = 2.82, p < .01. Binge drinking was also more likely on weekends, OR = 4.21, p < .0001, and days marked by elevated daily positive affect, OR = 1.60, p < .001. Binge drinking was not associated with concurrent depressive or general anxiety symptoms (p > .30). PTSD symptoms were not associated with use of cannabis or other substances (p > .06). Regarding the susceptibility model, following a binge drinking episode, participants reported elevated depressive symptoms, B = 0.34, p = .04, but no change in affect, PTSD symptoms, or general anxiety symptoms (p > .16). Results suggest that, beyond understanding who is at risk for binge drinking, fluctuations in PTSD severity clarify when students engage in binge drinking. (PsycInfo Database Record (c) 2021 APA, all rights reserved).