Background: In this study we aimed to develop a Spanish version of the Self-Efficacy/Social Support Scales for Activity for persons with Intellectual Disability (SE/SS-AID). Method: A cross-sectional study was carried out in a sample of 117 individuals with intellectual disability (ID). The SE/SS-AID scales were translated into Spanish and their psychometric properties analysed. Results: Confirmatory factor analysis showed acceptable fit (CFI = 0.94 for the SE-AID and 0.95 for the SS-AID), but RMSEA values outside of the optimal fit range (0.28 and 0.15, respectively). Cronbach's alpha was calculated for the SE-AID, SS-AID-family, SS-AID-staff, and SS-AID-peers scales, with results from 0.80 to 0.82. Conclusions: The scales show acceptable validity and reliability evidence. Use of this instrument will allow researchers to gain more knowledge about how personal and social factors influence participation in physical activity in Spanish-speaking people with ID.