The relationship between presleep worry and insomnia has been investigated in previous studies, but less attention has been given to the role of daytime worry and symptoms of insomnia. The aims of the current study were (a) to assess the psychometric properties of a novel scale measuring insomnia-specific worry during daytime and (b) to examine whether levels of daytime worry predict severity of insomnia symptoms. Participants (N = 353) completed the Insomnia Daytime Worry Scale (IDWS) and the Insomnia Severity Index. An explorative principal-axis factor analysis extracted two factors from the IDWS, accounting for 70.5% of the variance. The IDWS demonstrated good reliability. The total score of IDWS and both factors predicted levels of insomnia severity in two separate hierarchical regression analyses. This preliminary evidence suggests that the IDWS is a valid and reliable scale to measure daytime worry in insomnia.