The purpose was to investigate the psychometric properties of the Insomnia Catastrophizing Scale (ICS) including factorial validity and internal consistency as well as discriminative and convergent validity. Associations with sleep parameters and daytime impairment are also examined. Drawn from a randomly selected sample of the general population, 1615 participants completed a survey on insomnia-related nighttime and daytime symptoms, health outcomes and psychological processes, including the ICS. A one-factor solution was supported for both the nighttime catastrophizing (11 items) and daytime catastrophizing (6 items) subscales. Both subscales displayed high internal consistencies (α > 0.90) and accounted for 59.1-70.1% of the variance. The insomnia disorder group had significantly higher scores than participants without insomnia on the two subscales and on the individual items. Cutoffs were established for both subscales with acceptable sensitivity and specificity. Both subscales displayed adequate convergent validity with measures indexing worry, cognitive pre-sleep arousal and anxiety. The two subscales were also significantly associated with nighttime and daytime insomnia symptoms. The ICS is a reliable and valid scale for the assessment of insomnia-related catastrophizing. Future research is needed to examine the test-retest reliability and treatment sensitivity of the ICS.