Abstract: BACKGROUND: Sleep difficulties that cause some degree of distress or impairment, but do not meet full diagnostic criteria for insomnia (i.e., subthreshold insomnia) are associated with adverse mental and physical health outcomes among veterans, yet population-based longitudinal data examining the incidence and risk factors for this problem remain scarce. METHODS: A population-based sample of 1621 U S. veterans without subthreshold or clinical insomnia was followed over three years to examine how a broad range of baseline sociodemographic, psychological, and health-related risk factors predicted new-onset subthreshold insomnia (e.g., Insomnia Severity Index score range 8-14). RESULTS: A total of 315 veterans (weighted 19.9%, 95% confidence interval = 17.9-21.8%) developed new-onset subthreshold insomnia over the three-year follow up period. The strongest risk factors for subthreshold insomnia were being employed, neuroticism, loneliness, attentional difficulties, and chronic pain (10.1-22.3% explained variance). Among veterans with 0, 1, 2, and 3 of these risk factors, the predicted probability of subthreshold insomnia increased progressively from 11.6% to 18.3%, 27.6%, and 39.3%, respectively. CONCLUSIONS: Findings indicate that subthreshold insomnia is highly prevalent in the U.S. veteran population, with 1-in-5 veterans developing this problem over the study period. Key risk factors for subthreshold insomnia included employment status, neuroticism, and loneliness. Consistent with the 4P model of insomnia, results highlight the interplay of multiple predisposing, precipitating, perpetuating, and protective factors in the development of insomnia. These findings underscore the importance of early identification and targeted prevention strategies for at-risk veterans.