University adjustment is one of the most common life challenges that freshman encounter. There may be several individual differences that can either positively or negatively affect the adjustment process. Identifying the individual differences and variables that facilitate the adjustment process is significant to an understanding of students' adjustment difficulties and to hereby providing the opportunity for early intervention. For this reason, this study aims to investigate the direct effect of personality and humor styles on university adjustment and the indirect effect of humor styles on the relationship between personality and university adjustment. Behavioral activation and behavioral inhibition systems, postulated by Reinforcement Sensitivity Theory, were taken as a reference to measure personality characteristics. Humor styles were categorized in four different dimensions; self-enhancing, affiliative, self-defeating, and aggressive humor. Participants were 282 freshman students from a public university in Turkey. Of the participants 193 were female and 87 were male. Behavioral Activation-Inhibition Scale, Humor Styles Scale and University Life Scale were applied using a paper-pencil form. The results showed that while university adjustment was negatively predicted by behavioral inhibition, it was positively predicted by behavioral activation. Also, self-defeating humor predicted university adjustment negatively, while self-enhancing and affiliative humor predicted it positively. The results of the mediation analyses revealed that the relationship between behavioral activation and university adjustment was mediated partially by self-enhancing and affiliative humor styles. The mediational role of self-defeating humor in the relationship between behavioral inhibition and university adjustment was also examined, but the result showed that self-defeating humor had no mediational role. According to the results, BIS and self-defeating humor style seem to be risk factors for university adjustment whereas BAS, self-enhancing and affiliative humor styles facilitate this process. Students high in reward sensitivity use positive humor styles when adjusting to university, whereas difficulties in the adjustment process of students high in punishment sensitivity are not due to the use of malignant humor styles. For this reason, recognizing some personality characteristics as risk factors for university adjustment and enhancing positive humor styles as coping strategies may be helpful for dealing with the adjustment process.