Alcohol dependence (AD) and posttraumatic stress disorder (PTSD) are highly comorbid, yet limited research has focused on PTSD and daily drinking as they relate to self-appraised alcohol-related problems. In treatment contexts, patients’ appraisals of alcohol-related problems have implications for assessment, intervention strategies, and prognosis. This study investigated the moderating effect of within-person (daily symptoms) and between-person (overall severity) differences in PTSD on the association between daily drinking and same-day alcohol-related problems. Participants with comorbid AD and PTSD (N = 86) completed 1 week of Interactive Voice Recognition data collection, and logistic and γ-adjusted multilevel models were used to estimate odds and magnitude of self-appraised alcohol-related problems. Results revealed that both within-person and between-person PTSD moderated the association between number of drinks and severity of self-appraised problems. As within-person and between-person PTSD symptoms increased, there was a weaker association between number of drinks consumed and perceived alcohol-related problems. Contrasts further revealed that on nondrinking and light-drinking days, PTSD (both daily symptoms and overall severity) was positively associated with ratings of alcohol-related problems. However, PTSD was not associated with alcohol-related problems on heavier drinking days. In conclusion, more severe PTSD is associated with a less directly contingent relationship between drinking quantity and perceived alcohol-related problems. These findings suggest the importance of further investigations of this moderating effect as well as clinical treatment of comorbid AD and severe PTSD with functional analysis of drinking.