ObjectivesDespite the increasing prevalence of mindfulness-based interventions in sport, no context-specific instrument currently exists to measure mindfulness in sport. The Mindfulness Inventory for Sport (MIS) was devised using a three-stage approach, to measure one's ability to: (1) be aware of disruptive stimuli and their associated internal reactions; (2) adopt a non-judgmental attitude towards these stimuli and reactions; and (3) quickly refocus attention on goal-related cues. Method In stage 1, a pool of items was developed and assessed by six experts in the areas of mindfulness and instrument validation. In stage 2, exploratory factor analyses with data collected from undergraduate student-athletes (N = 370) resulted in a three-factor, 19-item version of the instrument. In stage 3, confirmatory analyses using structural equation modelling were conducted with a sample of elite athletes (N = 343). Results and conclusion A final 15-item three-factor version displayed an acceptable model fit, with little evidence of invariance demonstrated across sport type and partial invariance across gender. In addition, the subscales of the MIS displayed significant correlations with conceptually-related variables such as flow, worry, concentration disruption, and perfectionism.