Simple SummaryThe Positive Mental Health Scale (PMHS) is the most widely used instrument to measure positive mental health worldwide. However, there is no theoretical, clinical, or empirical knowledge regarding the psychometric properties of the PMHS in parents of children with cancer, indicating a significant gap in the literature. In this study, we aimed to investigate the psychometric properties of the PMHS in parents of children with cancer using a psychometric evaluation based on item response theory (IRT). To accomplish this purpose, an ex post facto study with a cross-sectional design and non-probability sampling was conducted to evaluate 623 parents of children with cancer at the National Institute of Health in Mexico City. The main findings indicate that the IRT graded response model validates the single latent trait model. The PMHS demonstrated adequate reliability and was found to be a valid, reliable, and culturally relevant instrument that can be used in parents of children with cancer.AbstractMental health is currently a public health issue worldwide. However, evidence is lacking regarding the validity of the instruments used to measure and assess positive mental health in specific populations. The objective of this study was to evaluate the psychometric properties of the PMHS using IRT. A cross-sectional retrospective study with non-probabilistic convenience sampling was conducted with 623 parents of children undergoing cancer treatment at the National Institute of Health in Mexico City. The participants responded to a battery of tests, including a sociodemographic questionnaire, the PMHS, Measurement Scale of Resilience, Beck Depression Inventory, Inventory of Quality of Life, Beck Anxiety Inventory, an interview regarding caregiver burden, and the World Health Organization Well-Being Index. PMHS responses were analyzed using Samejima’s graded response model. The PMHS findings indicated that the IRT-based graded response model validated the single latent trait model. The scale scores were independent of depression, anxiety, well-being, caregiver burden, quality of life, and resilience. The PMHS scores were associated with low subjective well-being. The PMHS findings reveal that from an IRT-based perspective, this scale is unidimensional and is a valid, reliable, and culturally relevant instrument for assessing positive mental health in parents of children with chronic diseases.