Sport rivalries have been shown to increase the emotional intensity of fans, which not only can lead to higher levels of interest and involvement but can also escalate negative fan behaviors based on in-group/out-group distinctions. This study represents the first use of an experimental economics approach in sport management to understand the behaviors of rival sports fans. Specifically, the classic behavioral economics experiment, the ultimatum game, was used to test the willingness of rival fans to make their out-group counterparts worse off. Using a $10 stake, proposers offered approximately 8.7% less to rival fans than to in-group fans, while the probability that a responder accepted an offer—holding constant offer size—was approximately 7% lower when the proposer was a rival. Team identification had no effect on offers or acceptances. Implications for understanding rivalry in sport are discussed, and advantages of behavioral economics for sport management research are noted.