Background: Individuals with intellectual disabilities have decreased leisure participation, affecting their health and well-being. Existing measures of leisure participation are primarily based on self- or proxy reports. This study describes the initial validity and reliability of Leisure Participation Observation (LPO), developed for individuals with ID. Method: Two judge panels (N = 10 each) completed a table of specifications for content validity. Reliability of the LPO was established on 38 individuals with moderate ID 25-45 years old. Results: This study established the content validity of the LPO. Additionally, weighted kappa showed moderate-high statistically significant inter-rater reliability (0.667-0.859, p < 0.01) and low-moderate, significant test-retest reliability, based on intraclass correlations (0.226-0.474, p < 0.05). Finally, results yielded moderate-high internal consistency ([alpha] = 0.74). Conclusions: The LPO shows promise as an observation tool for assessing leisure participation in the moderate ID population that may serve as the basis for developing intervention programmes.