Background The purpose of this study is to develop a Portuguese version of the Oral Health Impact Profile (OHIP-14) and validate it for people with mild intellectual disability (OHIP-14-MID-PT). Methods The Portuguese version of the questionnaire was drawn up from the original English version, following internationally defined guidelines. Interviews were conducted with 240 individuals living in (or attending) institutions of the central region of Portugal that are affiliated with Humanitas (Portuguese Federation for Intellectual Disability) to measure oral health related quality of life (OHRQoL). The interview also included a sociodemographic and oral health questionnaire followed by an intraoral examination. Two types of reliability were analyzed: test-retest (ICC) and internal consistency (Cronbach’s α, inter-item and item-total correlations). Convergent and divergent validities were also assessed, and a confirmatory factor analysis was performed using the maximum likelihood method. Results The OHIP-14-MID-PT presented high reliability (ICC = 0.999; Cronbach’s α = 0.922). The inter-item correlation coefficient ranged from 0.277 to 0.749, and the item-total correlation coefficient varied between 0.529 and 0.718. Lower OHIP-14-MID-PT total scores were significantly associated with: a self-perception of better oral health status (r = -0.545, p<0,001) and reduced need for dental treatment (U = 2366.5, p<0,001), more natural teeth (χ2 = 29.74, p<0,001) and better results in the clinical oral health index (COHI) (χ2 = 18.50, p<0,001); the results support the convergent and divergent validities of the questionnaire. Conclusions OHIP-14-MID-PT has proved to be a consistent, valid and reliable instrument with good psychometric properties to determine the impact of oral health on quality of life in adults with mild intellectual disabilities in Portugal.