Postoperative orthopedic patients are a high-risk group for receiving long-duration, large-dosage opioid prescriptions. Rigorous evaluation of state opioid duration limit laws, enacted throughout the country in response to the opioid overdose epidemic, is lacking among this high-risk group. We took advantage of Massachusetts' early implementation of a 2016 7-day-limit law that occurred before other statewide or plan-wide policies took effect and used commercial insurance claims from 2014-2017 to study its association with postoperative opioid prescriptions greater than 7 days' duration among Massachusetts orthopedic patients relative to a New Hampshire control group. Our sample included 14 097 commercially insured, opioid-naive adults aged 18 years and older undergoing elective orthopedic procedures. We found that the Massachusetts 7-day limit was associated with an immediate 4.23 percentage point absolute reduction (95% CI, 8.12 to 0.33 percentage points) and a 33.27% relative reduction (95% CI, 55.36% to 11.19%) in the percentage of initial fills greater than 7 days in the Massachusetts relative to the control group. Seven-day-limit laws may be an important state-level tool to mitigate longer duration prescribing to high-risk postoperative populations.