Persistent smoking may cause adverse outcomes among patients with cancer. Many cancer centers have not fully implemented evidence-based tobacco treatment into routine care. To determine the effectiveness of sustained telephone counseling and medication (intensive treatment) compared with shorter-term telephone counseling and medication advice (standard treatment) to assist patients recently diagnosed with cancer to quit smoking. This unblinded randomized clinical trial was conducted at Massachusetts General Hospital/Dana-Farber/Harvard Cancer Center and Memorial Sloan Kettering Cancer Center. Adults who had smoked 1 cigarette or more within 30 days, spoke English or Spanish, and had recently diagnosed breast, gastrointestinal, genitourinary, gynecological, head and neck, lung, lymphoma, or melanoma cancers were eligible. Enrollment occurred between November 2013 and July 2017; assessments were completed by the end of February 2018. Participants randomized to the intensive treatment (n = 153) and the standard treatment (n = 150) received 4 weekly telephone counseling sessions and medication advice. The intensive treatment group also received 4 biweekly and 3 monthly telephone counseling sessions and choice of Food and Drug Administration-approved cessation medication (nicotine replacement therapy, bupropion, or varenicline). The primary outcome was biochemically confirmed 7-day point prevalence tobacco abstinence at 6-month follow-up. Secondary outcomes were treatment utilization rates. Among 303 patients who were randomized (mean age, 58.3 years; 170 women 56.1%), 221 (78.1%) completed the trial. Six-month biochemically confirmed quit rates were 34.5% (n = 51 in the intensive treatment group) vs 21.5% (n = 29 in the standard treatment group) (difference, 13.0% 95% CI, 3.0%-23.3%; odds ratio, 1.92 95% CI, 1.13-3.27; P < .02). The median number of counseling sessions completed was 8 (interquartile range, 4-11) in the intensive treatment group. A total of 97 intensive treatment participants (77.0%) vs 68 standard treatment participants (59.1%) reported cessation medication use (difference, 17.9% 95% CI, 6.3%-29.5%; odds ratio, 2.31 95% CI, 1.32-4.04; P = .003). The most common adverse events in the intensive treatment and standard treatment groups, respectively, were nausea (n = 13 and n = 6), rash (n = 4 and n = 1), hiccups (n = 4 and n = 1), mouth irritation (n = 4 and n = 0), difficulty sleeping (n = 3 and n = 2), and vivid dreams (n = 3 and n = 2). Among smokers recently diagnosed with cancer in 2 National Cancer Institute-designated Comprehensive Cancer Centers, sustained counseling and provision of free cessation medication compared with 4-week counseling and medication advice resulted in higher 6-month biochemically confirmed quit rates. However, the generalizability of the study findings is uncertain and requires further research. ClinicalTrials.gov Identifier: NCT01871506.