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length: 150 words Abstract Surprise signals a discrepancy between predicted and observed outcomes. It is theorized to segment the flow of experience into discrete perceived events, drive affective experiences, and create particularly resilient memories. However, the ability to precisely measure naturalistic surprise has remained elusive. We used advanced basketball analytics to derive a quantitative measure of surprise and characterized its behavioral, physiological, and neural effects on human subjects observing basketball games. We found that surprise served to segment ongoing experiences, as reflected in subjectively perceived event boundaries and shifts in neocortical neural patterns underlying belief states. Interestingly, these effects differed by whether surprising moments contradicted or bolstered current predominant beliefs.Surprise also positively correlated with pupil dilation, processing in subcortical regions associated with dopamine, game enjoyment, and, along with these physiological and neural measures, long-term memory. These investigations support key predictions from event segmentation theory and extend theoretical conceptualizations of surprise to real-world contexts.