During college years, which are known as a stressful time, students may often face stress personally, socially, academically, economically, and so forth in various areas of life. One of the important sources that students use to cope with stress is social support. Students can cope with stress easier via the support they receive from their friends or family. While social support is seen as a protective mechanism against stress, some psychological factors may have increasing or decreasing effects on this support. Positive and negative affects, two opposing indicators of individual psychological adjustment, may function differently in this context. In the current study, the moderator roles of positive and negative affect on the relationship between perceived social support and stress in college students was investigated. The sample consisted of 479 undergraduate students from the faculty of education in a public university. Data was collected using the Multidimensional Scale of Perceived Social Support, the Perceived Stress Scale, and the Positive and Negative Affect Scale. In data analysis, hierarchical multiple regression analysis based on the steps of Baron and Kenny’s (1986) moderating model was used. The research findings indicate that negative affect has a moderator role in the relationship between perceived social support and stress, whereas positive affect does not have a similar function. Accordingly, as negative affect increases, the positive effect of social support on perceived stress decreases.