Background: To characterize cannabis use among cancer patients, we aimed to describe 1) patterns of cannabis use across multiple cancer sites; 2) perceived goals, benefits, harms of cannabis; and 3) communication about cannabis. Methods: Patients with 9 different cancers treated at Memorial Sloan Kettering Cancer Center between March and August 2021 completed an anonymous online survey eliciting cannabis use, attitudes toward cannabis, and communication about cannabis. Multivariable logistic regression estimated the association of cancer type and cannabis use, adjusting for sociodemographic characteristics and prior cannabis use. Results: Among 1258 respondents, 83% believed there were benefits to cannabis use for cancer patients (N = 1032), and 55% (N = 666) believed there were potential harms. Thirty-one percent (N = 385) used cannabis after diagnosis (range: 25% among lung cancer to 59% among testicular cancer patients). Fifty-two percent of those using cannabis after diagnosis (N = 197) reported use in the past month. Characteristics associated with cannabis use after cancer diagnosis included younger age, lower education level, cancer type, and cannabis use in the year prior to diagnosis. In multivariable analysis, compared to lung cancer patients, gastrointestinal cancer patients were more likely to use cannabis (odds ratio [OR] 2.64, 95% confidence interval [CI] 1.25-5.43). Cannabis use in the year prior to diagnosis was strongly associated with cannabis use after diagnosis (OR 19.13, 95% CI 11.92-30.72). Among users, the most common modes of ingestion were eating (30%, N = 115), smoking (26%, N = 99), and pills/tincture (23%, N = 87). The most commonly endorsed reasons for use included difficulty sleeping (48%, N = 182); stress, anxiety, or depression (46%, N = 175); and pain (42%, N = 160); 29% (N = 123) reported using cannabis for recreation, and 5% (N = 17) used for treatment or cure of their cancer. Among respondents who used cannabis to improve symptoms, 70-90% reported improvement; < 5% reported that any symptom worsened. Among cannabis users, 48% (N = 185) discussed cannabis with an oncology provider, and cannabis products were most commonly acquired through friends, family members, or members of the community (47%, N = 176). Conclusions: In a large sample of recently or currently treated patients with a range of cancers, almost a third of patients used cannabis after their cancer diagnosis, most commonly edibles. Patients who used cannabis largely did so for symptom relief rather than with the goal of treating or curing their cancer. Limited communication about cannabis and acquisition of cannabis outside the medical setting suggests that oncology providers may not know about their patients’ cannabis use. To improve informed decision making about cannabis use during cancer care, research to determine benefits and harms of cannabis use is needed alongside enhanced clinical communication.