Assessing quality of life (QoL) is important to provide personalized and individualized support plans with the purpose of improving personal outcomes. Based on the conceptual model of QoL, the aim of this study was to assess the congruence between the perceptions of institutionalized individuals with intellectual and developmental disabilities (IDD) and the perception of a third party, regarding QoL. Forty-two individuals participated in this study, including twenty-one with mild to severe IDD and their family members/caregiver/reference technician, who responded to the Personal Outcomes Scale (Portuguese version). Significant differences (p< 0.05) were found between reports in the personal development (t= −2.26;p= 0.024), emotional well-being (t= −2.263;p= 0.024), physical well-being (t= −2.491;p= 0.013) and total QoL (t= −2.331;p= 0.02). The results further show that most third-party reports tend to undervalue the QoL of the individual with IDD, and that there is no congruence in any of the QoL domains. The inclusion of self-reports in the QoL assessment is important. In addition to the assessment of third-party reports, the process of making decisions appropriate to the context and individual characteristics is equally important. On the other hand, the inclusion of third-party reports is an opportunity to promote communication among all stakeholders, recognize and discuss differences, and promote QoL, not only of individuals with IDD, but also of families.