Research evaluating phonics reading programs for children with severe intellectual disabilities (ID) is limited. The current study investigated whether using an online reading program (Headsprout® Early Reading; HER) as supplementary reading instruction for children with a severe ID leads to improvements in reading skills as compared to children not receiving this additional instruction. Fifty-five children from a special school were randomly allocated into the HER group or a waiting list control group. For six months, children in the intervention group received HER as supplementary instruction, whereas children in the control group received only ‘reading as usual’ teaching. Pre- and post-intervention tests on standardised reading measures were conducted. Analysis of data from outcome measures indicated that the HER group made improvements at post-intervention in comparison with the control group, with medium effect sizes on two domains from the main outcome measure. These results support the case for a larger research trial of HER for children with severe ID.