Professional sports have unwritten, but nevertheless well-appreciated and behavior-guiding rules that have come to be viewed as obligatory and subject to enforcement. This essay uses such unwritten rules from professional sports as novel lenses through which to view afresh, and more richly to appreciate, customary international law (CIL) rules’ nature, origins, and evolution. Though much CIL scholarship has presumed that merely tradition and self-interest have empowered CIL’s legal force, unwritten rules from professional sports demonstrate that emergent and even unacknowledged norms can and do necessitate widely accepted actions and penalties. This essay begins with a discussion of CIL’s literature, distinct nature, and methodological challenges. It turns next to a review and analysis of four prominent, unwritten rules drawn from professional sports. Finally, based on that analysis, it identifies particular lessons of salience for students of CIL. This essay also specifies the broader implications for CIL of professional sports’ unwritten-but-obligatory rules.