The present case study investigated the probable effect of applying a humanoid robot as a teacher-assistant for learning and retention of English vocabulary among 10 Iranian foreign language learners with intellectual disabilities, more specifically Down syndrome. This method is a new trend in using technology to teach classes called Robot Assisted Language Learning (RALL). To this end, participants were divided into 2 groups of five, one with the help of the robot (RALL) and one without (non-RALL). Both groups received the same lessons to learn 40 English vocabulary in 8 sessions. To measure the results, three English tests were run at different times during the teaching sessions (a pre-test, post-test, and a delayed post-test). The independent sample t-test on gain scores from pre-test to post-test revealed that there was a significant difference between the gains of the RALL group (M = 12.40) and the non-RALL group (M = 8.40), which indicates that RALL instruction had a large effect on the participants' English vocabulary improvement. Moreover, while both RALL and non-RALL groups retained the taught vocabulary, the RALL group showed a more satisfying result. The findings of this study demonstrate the positive effects of using a humanoid robot to facilitate vocabulary learning and retention among people with Down syndrome who have issues with both language learning and verbal short-term memory and need to learn through visual activities, gestures, and games. This study could be a starting point for a new line of research in second or foreign language instruction specific to people with Down syndrome.