The purpose of the study was to evaluate whether students with intellectual disabilities (ID) can improve their arithmetic skills by participating in an arithmetic intervention programme, theoretically founded on explicit instruction (EI) and administered via an application developed for tablet computers. The intervention study used a randomised controlled trial design (RCT) (n = 30, aged 10-16, 13 females) and lasted for up to 12 weeks. The results show that the intervention group significantly improved in arithmetic fact fluency compared to the controls and the effects remained six months after the intervention. The effects were larger for subtraction than for addition, and this difference remained six months later. These results suggest that mathematics applications based on explicit instruction can be an effective way of teaching arithmetic facts to youth with mild ID.