Background: The ARMIDILO-S is advocated as a promising tool for assessing dynamic risk factors in sex offenders with intellectual disabilities (SOIDs). However, research remains scarce. The present study aimed to further validate this instrument in SOIDs. Method: The study prospectively followed 38 SOIDs for up to one year to test the accuracy of the ARMIDILO-S in predicting violent and sexual incidents. Results: Overall predictive accuracy was moderate to high. The ARMIDILO-S further showed excellent prospective qualities in identifying both high-risk and low-risk offenders for violence. Regarding sexual offending, it was only good at prospectively detecting low-risk individuals. Conclusions: This study provided further evidence of the good predictive validity of the ARMIDILO-s in predicting future sexual and violent incidents in SOIDs. More research in preferably larger samples as well as field validity studies are recommended.