IMPORTANCE: Polypharmacy and inappropriate drug regimens are major health concerns among older adults. Various interventions focused on medication optimization strategies have been carried out, but the effect on patient-relevant outcomes remains uncertain. OBJECTIVE: To investigate the effect of clinical geriatric assessments and collaborative medication reviews by geriatrician and family physician (FP) on health-related quality of life and other patient-relevant outcomes in home-dwelling older patients receiving polypharmacy. DESIGN, SETTING, AND PARTICIPANTS: Cluster randomized, single-blind, clinical trial. Norwegian FPs were recruited from March 17, 2015, to March 16, 2017, to participate in the trial with their eligible patients. Participants were home-dwelling patients 70 years or older, using at least 7 medications regularly, and having their medications administered by the home nursing service. Patients in the control group received usual care. Randomization occurred at the FP level. A modified intent-to-treat analysis was used. INTERVENTION: The intervention consisted of 3 main parts: (1) clinical geriatric assessment of the patients combined with a thorough review of their medications; (2) a meeting between the geriatrician and the FP; and (3) clinical follow-up. MAIN OUTCOMES AND MEASURES: The primary outcome was health-related quality of life as assessed by the 15D instrument (score range, 0-1; higher scores indicate better quality of life, with a minimum clinically important change of ±0.015) at week 16. Secondary outcomes included changes in medication appropriateness, physical and cognitive functioning, use of health services, and mortality. RESULTS: Among 174 patients (mean [SD] age, 83.3 [7.3] years; 67.8% women; 87 randomized to the intervention group and 87 randomized to the control [usual care] group) in 70 FP clusters (36 intervention and 34 control), 158 (90.8%) completed the trial. The mean (SD) 15D instrument score at baseline was 0.708 (0.121) in the intervention group and 0.714 (0.113) in the control group. At week 16, the mean (SD) 15D instrument score was 0.698 (0.164) in the intervention group and 0.655 (0.184) in the control group, with an estimated between-group difference of 0.045 (95% CI, 0.004-0.086; P = .03). Several secondary outcomes were also in favor of the intervention. There were more drug withdrawals, reduced dosages, and new drug regimens started in the intervention group. CONCLUSIONS AND RELEVANCE: This study's findings indicate that, among older patients exposed to polypharmacy, clinical geriatric assessments and collaborative medication reviews carried out by a geriatrician in cooperation with the patient's FP can result in positive effects on health-related quality of life. TRIAL REGISTRATION: ClinicalTrials.gov identifier: NCT02379455.