This study examined whether living in a Medicaid-expanded state or having health insurance was associated with receipt of smoking cessation services or smoking behaviors among substance use disorder (SUD) treatment clients. In 2015 and 2016, 1702 SUD clients in 14 states were surveyed for health insurance status, smoking cessation services received in their treatment program, and smoking behaviors. Services and behaviors were then compared by state Medicaid expansion and health insurance status independently. Clients in Medicaid-expanded states were more likely to be insured (89.9% vs. 54.4%, p < 0.001) and to have quit smoking during treatment (AOR = 3.77, 95% CI = 2.47, 5.76). Insured clients had higher odds of being screened for smoking status in their treatment program and making quit attempts in the past year. Medicaid expansion supports greater health insurance coverage of individuals in SUD treatment and may enhance smoking cessation.