Objective: This is the first study to examine self-reported seasonal differences in the severity of ADHD symptoms in adults from the general population. Method: Data were analyzed from N = 5,303 respondents participating in the second wave of the Netherlands Mental Health Survey and Incidence Study–2, a population-based study on mental health. ADHD symptoms were assessed using the Adult ADHD Self-Report Scale Screener. As indicators of the severity of ADHD symptoms, the total ADHD symptom score and inattention and hyperactivity subscale scores were examined. Results: Compared with participants who were assessed in autumn, total ADHD and inattention subscale scores were significantly higher among participants who were assessed in spring or summer; the hyperactivity subscale score was significantly higher in spring. Conclusion: We found seasonal variations in the severity of ADHD symptoms, which was highest in those assessed in spring and summer. Researchers should be aware of this in the diagnostic process.