Importance: Forty percent to 75% of community-dwelling older adults are not able to adhere to their medication routine. A medication management assessment can correctly identify the reasons for nonadherence and the barriers contributing to it. Objective: To further develop the HOME–Rx, an in-home medication management assessment, by modifying scoring metrics, improving clinical utility, and establishing psychometric properties. Design: In Phase 1, the scoring metrics were modified, and the clinical procedures were evaluated. In Phase 2, the psychometric properties were established. Setting: The homes of older adults. Participants: Older adults who took three or more medications, managed their own medications, and lived in their own home were eligible. Older adults with cognitive impairment were ineligible. Outcomes and Measures: We assessed concurrent validity with the Performance Assessment for Self-Care Skills (PASS) and Medication Management Instrument for Deficiencies in the Elderly (MedMaIDE) and established interrater reliability. Results: The PASS was positively correlated with the HOME–Rx Performance and Safety subscales; the MedMaIDE was negatively correlated with the HOME–Rx Performance subscale and positively correlated with the Barriers subscale. Interrater reliability was excellent (ICCs = .87–1.00). Conclusions and Relevance: All relationships were as predicted: The HOME–Rx is a valid and reliable performance-based assessment that provides clinicians and researchers with a measure of older adults’ actual medication management ability in the home using their medications. The results can potentially be used to guide treatment planning and improve medication management. What This Article Adds: Occupational therapy practitioners can use the HOME–Rx to adequately determine performance problems, safety concerns, and environmental barriers and potentially to guide treatment planning and improve medication management for older adults.