Purpose: The purpose of this study was to investigate factors associated with high school physical education (PE) teachers' adoption of a supplemental online instructional system. Method: Semistructured, open-ended phone interviews with 28 high school PE teachers were used as the primary data collection method. All teachers were using or had used a supplemental online instructional system at the time of the study. The Unified Theory of Acceptance and Use of Technology (UTAUT) guided the directed content analysis. Results: Four main categories were generated, including perceived programmatic, instructional, and inclusivity improvements; minimal personal and student usage effort; school and curriculum provider support facilitated use; and administrators' dictated long-term use. Discussion/Conclusion: The results aligned well with the UTAUT and served to situate the theory within the secondary PE context. The participants' perceptions and experiences were also contradictory to much of the current research on teachers' technology adoption in PE and K-12 education, more generally.