Purpose: To identify the role of content specificity in moderating situational interest and its relationship with physical activity (PA), we examined the between- and within-content relations of situational interest and PA in an urban elementary physical education context using fitness and basketball as comparative contents. Method: Fourth and fifth graders (N = 125) completed questionnaires and wore pedometers to assess situational interest and PA across a fitness unit and a basketball unit, respectively. Results: Based on our confirmatory factor analyses and fit indexes, situational interest, and PA demonstrated strong content specificity. Strengths of associations between the two units differed by situational interest dimensions and PA (r = 0.50). Within-content interrelations of these dimensions and PA were not consistent (X[superscript 2] Decrease = 3.40, p < 0.05). Conclusion: Our findings suggest that content specificity in physical education plays a significant role in moderating students' motivation and its function on PA.