Background: Mentalising and stress regulation pose challenges for adults with mild to borderline intellectual disabilities (MBID), emphasising the importance of an intervention program. The study examined the effectiveness and social validity of the serious game 'You & I' in enhancing mentalising and stress regulation among adults with MBID. Method: A randomised controlled superiority trial with experimental and waitlist-control groups was conducted with 159 adults with MBID (M[subscript age] = 36) at baseline, post-test, and follow-up. Analyses investigated the effects on aspects of mentalising, stress regulation, and social validity. Results: The experimental group showed decreased stress from negative interpersonal relations, while the control group experienced increased stress (d = 0.26). There were no significant effects on mentalising, but positive user expectations and experiences were reported. Conclusions: This initial study on 'You & I' provides limited evidence of its effectiveness for people with MBID, warranting further examination of the potential of serious games.