Background: This paper presents a description of the development and psychometric properties of a self-report instrument for the assessment of sexual behaviour and concerns of people with mild intellectual disabilities (SEBECOMID-S). Methods and procedures: The study included 281 people with mild intellectual disabilities. The psychometric properties were examined through exploratory factorial analysis, descriptive statistics, and reliability indices. Results: The exploratory factor analyses offered a structure with three factors: concern about the appropriateness of their sexual behaviour, sexual practices performed, and safe sex practices. The model presents an excellent fit (?[superscript 2]/df = 1.10, RMSEA = 0.019, CFI = 0.997, TLI = 0.995, and SRMR = 0.065). General test reliability was good ([alpha] = 0.77, [omega] = 0.76). Conclusions: SEBECOMID-S is a valid and reliable tool to obtain objective information about the sexual behaviour and concerns of people with mild intellectual disabilities. The use of this instrument will make it possible to adjust their training to their real experiences, making it more effective.