OBJECTIVES: To describe and examine trends in polypharmacy according to age in Japan from 2010 to 2016. DESIGN: Retrospective observational study. SETTING: Outpatient settings. PARTICIPANTS: Japanese individuals aged 20 and older. MEASUREMENTS: We analyzed pharmacy claims data that the Japanese Ministry of Health, Labor, and Welfare provided in the Survey of Medical Care Activities in Public Health Insurance from 2010 to 2016. The use of 5 or more oral prescription medications per month was defined as polypharmacy and of 10 or more as excessive polypharmacy. Regression analysis was used to estimate trends in polypharmacy with annual percentage changes. Using number of medications (polypharmacy vs excessive polypharmacy), trends in polypharmacy and crude and age-adjusted rates of polypharmacy per 1,000 persons were calculated according to year and age group (20-34, 35-49, 50-64, 65-79, ≥ 80). RESULTS: We analyzed 240 million pharmacy claims data. The age-adjusted monthly prevalence rate of polypharmacy increased from 85.2 to 93.8 per 1,000 persons per month and of excessive polypharmacy from 13.6 to 14.0 per 1,000 persons per month from 2010 to 2016 in the entire study population. The highest rate of polypharmacy (per 1,000 persons) was observed in 2016 in those aged 80 and older (326.8), followed by those aged 65 to 79 (167.3). The polypharmacy rate increased by 6.3% (95% confidence interval (CI)=4.0-8.7) per year from 2010 to 2012, then decreased by 0.7% (95% CI=-1.3-0.0) per year from 2012 to 2016. The rate of excessive polypharmacy increased by 4.5% (95% CI=1.1-8.0) per year from 2010 to 2013 and then decreased by 3.7% (95% CI=-6.7 to -0.6) per year from 2013 to 2016. CONCLUSION: The overall trend of polypharmacy in Japan increased during the study period, although the increase ceased in 2013 and then declined from 2013 to 2016. Policy changes in Japan might be responsible for some of the changes. J Am Geriatr Soc 66:2267-2273, 2018.