Individual differences in intolerance of uncertainty are hypothesized to play a role in certain difficulties and distress in individuals with psychosis. However, the few studies that have directly explored this have yielded mixed results, which are difficult to interpret because measures of intolerance of uncertainty have not been formally validated for use with this population. To this end, the primary goal of the present study was to validate a brief (9-item) version of the Intolerance of Uncertainty Scale (Freeston et al. Personality and Individual Differences, 17, 791–802 1994) in an outpatient sample of 252 adults with psychosis. Results showed strong evidence for reliability (internal consistency and 6-month test-retest), as well as concurrent, predictive, and discriminant validity. Scores from the brief IUS were significantly correlated with emotional distress (e.g., anxiety, depression, hopelessness), quality of life, global functioning, and delusions, but not hallucinations. Further, these scores predicted changes in levels of general anxiety over time. Notably, intolerance of uncertainty was associated with symptoms/distress and functioning above and beyond pertinent cognitive variables (e.g., defeatist beliefs, asocial beliefs), suggesting this brief scale has incremental validity. Overall, results suggest it is a promising tool for future research on psychosis, and possibly clinical work with this population.