Background: Self-determination is a fundamental component influencing autonomy, well-being, and societal participation among individuals with intellectual and developmental disabilities (IDD). However, in India, the absence of culturally adapted and psychometrically validated tools poses significant barriers to accurately assessing selfdetermination capacities and supporting personcentred interventions. Objective: This study aimed to develop and validate the Self-Determination Scale for Individuals with Intellectual and Developmental Disabilities (SDS-IDD), an assessment tool tailored to the Indian sociocultural context. Methods: Following a rigorous multi-phase process, the SDS-IDD was developed through literature review, expert panel consultations, and focus group discussions involving stakeholders, including adults with IDD, caregivers, and special educators. A sample of 100 adults with mild to moderate intellectual and developmental disabilities (IDD) from urban and semi-urban regions of India was recruited for field testing. Psychometric evaluation included assessments of internal consistency using Cronbach’s alpha, test-retest reliability over a two-week interval, and construct validity through exploratory factor analysis (EFA). Results: The SDS-IDD demonstrated high internal consistency (Cronbach’s α = 0.86) and strong test-retest reliability (r = 0.82, p < 0.001). Factor analysis revealed a robust three-factor structure encompassing Autonomy, Goal-Setting, and Self-Advocacy domains, explaining 68.7% of the total variance. Convergent validity was supported by moderate positive correlations with adapted versions of The Arc’s SelfDetermination Scale. Conclusion: The SDS-IDD emerges as a culturally sensitive, reliable, and valid instrument for measuring selfdetermination among Indian adults with IDD. Its application holds significant potential for enhancing individualised support planning, program evaluation, and disability policy development in India. Future research should focus on broader regional adaptations and longitudinal validation to strengthen its utility across diverse Indian contexts.