Despite close links between the tenets of self-determination theory (SDT) and the concept of interest, few studies have closely investigated their integration in the area of physical Activity (PA). This study explored the nature of situational interest and need support. Surveys were administered at two data points (4-week window) to college students (n = 347) enrolled in PA classes. Confirmatory factor analysis results point to needs support as a separate factor associated with situational interest. Structural equation model analysis presented an acceptable fit, X[superscript 2] = 195.825 df = 83, p < 0.05, CFI = 0.953, TLI = 0.932, RMSEA = 0.063, SRMR = 0.049. Situational interest and need support are separate factors. Situational interest and optimal challenge have an inconsistent relationship with situational interest. Overall findings support the integration of interest theory and SDT, with clearer integration of personal interest into SDT compared to situational interest.