PURPOSE: Did the creation of an urgent care clinic specifically for patients with cancer affect emergency department visits among adults newly diagnosed with cancer? PATIENTS AND METHODS: We applied an interrupted time series analysis to adjusted monthly emergency department visits made by adults age 18 years or older who were diagnosed with cancer between 2009 and 2016 at a comprehensive cancer center. Cancer registry patients were linked to a longitudinal regional database of emergency department and hospital visits. Because the urgent care clinic was closed on weekends, we took advantage of the natural experiment by comparing weekend visits as a control group. Our primary outcome was emergency department visits within 180 days after a cancer diagnosis, compiled as adjusted monthly rates of emergency department visits per 1,000 patient-months. We analyzed subsequent hospitalizations as a secondary outcome. RESULTS: The rate of weekday emergency department visits was increasing at a rate of 0.43 visits (95% CI, 0.29 to 0.57 visits) per month before May 2012, then fell in half to a rate of 0.19 visits (95% CI, 0.11 to 0.28 visits) per month ( P = .007) after the urgent care clinic was established. In contrast, the weekend visit rate was growing at a rate of 0.08 visits (95% CI, −0.03 to 0.19 visits) per month before May 2012 and 0.05 (95% CI, −0.02 to 0.13 visits; P = .533) afterward. By the end of 2016, there were 15.3 fewer monthly weekday emergency department visits than expected ( P = .005). Trends in weekday hospitalizations were not significantly changed. CONCLUSION: Although only one in eight emergency department–visiting patients also used the urgent care clinic, the growth rate of emergency department visits fell by half after the urgent care clinic was established.