Caregiving for mental health among people with intellectual disabilities (IDs) in the ID services was reported as insufficient. The purposes of this study were to investigate five types of peer victimization (PV) experiences among adults with ID using ID services, and to gain a deeper understanding of the influence of PV experience on adults with ID’s long-term mental health status. A one-year longitudinal follow-up study was conducted from eight long-term care ID services (n = 176). Logistic regression analysis was applied to variables comprising personal characteristics, various types of PV experience and polyvictimization to predict period prevalence of psychiatric symptoms. The data indicated that nearly one-third of individuals with ID experienced at least one psychiatric symptom. The three most common psychiatric symptoms prevalent after one year were adjustment disorder, anxiety disorder, and somatoform disorder. Over the 1-year study period, approximately 40% of adults with ID reported experiencing PV. The most frequently reported types of PV were physical force (26%) and verbal victimization (22%). Polyvictimization was experienced by approximately a quarter of adults with ID. The findings suggest that PV is a common experience among adults in ID services. Thus, for a clearer understanding of mental health risks, caregivers should pay attention to adults with ID who experienced PV.