Background: The evidence base for psychological treatments for autism and mood disorders in people with moderate to severe intellectual disabilities (ID) is limited. Recent promising robot-based innovations in mental health care suggest that robot-based animal assisted therapy (AAT) could be useful to improve social skills and mood in people with moderate to severe ID. This study explores the efficacy of the innovative therapeutic robot seal Paro on alertness and mood in adults with moderate to severe ID. Method: Five N = 1 studies were performed in adults (59-70 years) with moderate to severe ID. During a control phase of four weeks, participants could interact with a plush seal, which was replaced by the robot seal Paro during an equally long treatment phase. Results: In one participant, alertness and mood, as rated by supervisors, improved during the treatment phase as compared to the baseline phase. The other four participants did not improve on either of the outcome measures. Conclusion: We conclude that robot-based AAT does not have clear beneficial effects on alertness and mood in adults with moderate to severe ID, but that positive interactions with the robot seal could be of therapeutic value in itself.