Objective: Attention-deficit/hyperactivity disorder (ADHD) is linked prospectively to cannabis; however, no study has examined mechanisms underlying this comorbidity. We examined 5 cannabis motives (i.e., coping with negative affect, sleep, social anxiety, perceived low risk of cannabis, and altered perception) as mediators of the prospective ADHD-cannabis relation. Method: Veterans reporting lifetime cannabis use (N=361; 93% male; 80% White) completed three semiannual assessments. Prospective mediation models using structural equation modeling analyzed the indirect effects of baseline ADHD symptoms on 12-month cannabis use and problems via each motive at 6 months. Zero-inflated negative binomial models were employed for both manifest outcomes and ADHD symptoms and motives were each modeled as 1-factor latent variables. Results: Sleep motives was a robust mediator for cannabis use frequency in single mediator models and was marginally significant when examined simultaneously with other motives after accounting for baseline cannabis use, demographics, other substance use, and other psychopathology. Coping with negative affect was the only significant mediator of ADHD symptoms and subsequent cannabis problem severity. Conclusion: Among veterans with ADHD symptoms, sleep disturbance is a salient motive for cannabis use, whereas coping with negative affect is a proximal predictor of cannabis problems. Findings support addressing sleep disturbance in adults with ADHD symptoms and converge with extant literature demonstrating robust associations between coping motives and substance use problems.