This study investigated whether affective scenarios (negative, neutral, or positive) influence the intention to engage in physical activity, and whether this relationship is mediated by expected joy. The study was based on the Affective Gradient Hypothesis, which proposes that behavior is driven by current feelings and anticipated future affect. Participants (N = 264) were randomly assigned to one of three scenario groups. A repeated measures ANOVA revealed a significant interaction effect for positive affect, indicating that the positive scenario produced a significantly larger boost in positive affect compared to the negative scenario. A mediation analysis showed that expected joy significantly mediated the relationship between the scenario group and physical activity intention. An exploratory moderated mediation analysis found no evidence that individual differences in the tendency to approach (rather than avoid) physical effort moderated this indirect pathway. Altogether, the findings suggest that interventions aimed at promoting exercise should focus on increasing expected enjoyment, as this serves as a key psychological mechanism linking emotional context to behavioral intention.