A few studies have indicated that adults with Autism Spectrum Disorder (ASD) and Attention-Deficit/Hyperactivity Disorder (ADHD) are more inclined to ruminate than adults in the general population. The present study examined whether subclinical ASD symptoms including difficulties in social interaction and attention to detail and ADHD symptoms that were composed of inattention and hyperactivity-impulsivity were related to depressive rumination in undergraduate students. This study also examined whether rumination is a mediating factor in the relationship of ASD and ADHD symptoms with depression. Non-clinical undergraduate students (N = 294) in Japan completed the Autism-Spectrum Quotient, Adult ADHD Self-Report Scale, interpersonal conflict subscale of the Interpersonal Stress Event Scale, Ruminative Responses Scale, and Beck Depression Inventory-Second Edition. Structural equation modeling indicated that hyperactivity-impulsivity was positively associated with rumination both directly and indirectly via interpersonal conflict, and that attention to detail and inattention were directly and positively related to rumination. The significant relationship between difficulties in social interaction and rumination disappeared after controlling for the influence of depression. These findings indicated that one pathway through which hyperactivity-impulsivity symptoms prolong rumination might be through the increase in interpersonal conflict. In addition, it is possible that cognitive inflexibility, academic difficulties, and adverse driving outcomes caused by attention to detail, inattention as well as hyperactivity-impulsivity may lead to rumination. Moreover, attention to detail, inattention, and hyperactivity-impulsivity indirectly increased depression via rumination, indicating that rumination is an important mediator in the relationship of subclinical ASD and ADHD symptoms with depression.