Background: Psychotic-like experiences (PLEs) are low-intensity subclinical phenomena, often transient in nature. The etiology of PLEs primarily involves neurodevelopmental changes, trauma exposure, and maladaptive coping styles. Attention-Deficit/Hyperactivity Disorder (ADHD) is considered to be one of the factors that increase the risk of future psychosis. Furthermore, ADHD symptoms predict a heightened incidence of traumatic experiences, ruminative thoughts, and negative affect (NA). This present study examines whether rumination and NA mediate the relationship between ADHD symptoms and PLEs and whether trauma experiences moderate these pathways. Methods: A total of 188 participants (72% female) aged 18–35 completed questionnaires assessing ADHD symptoms and traumatic experiences and took part in a seven-day experience sampling method (ESM) procedure, completing ratings of PLEs experiences, the intensity of ruminations, and NA. Results: Correlation analysis showed significant relationships between all tested variables. Serial mediation analysis revealed a significant indirect effect of rumination and NA in the link between ADHD symptoms and PLEs. There was no significant impact of trauma experience in this relationship. Conclusions: Our study underscores the important role of rumination and NA in the co-development of ADHD symptoms and PLEs. Future research should consider investigating the intra-individual dynamics of ADHD and trauma using ecologically valid research methods in the context of PLEs to better understand these complex relationships.