Aims: This study aimed to assess the habits and knowledge of cancer patients regarding the use of herbal medicines and dietary supplements in cancer patients receiving immune checkpoint inhibitors (ICI). Methods: The data of 181 cancer patients who were over 18 years old and received ICIs were collected. The usage patterns, anticipated benefits and harms, and sources of supply were evaluated by filling researcher-prepared forms. Results: Most patients did not use any kind of herbal medicine (91.2%) or dietary supplements (75.9%) during their immunotherapy. Boosting the immune system is the primary motivation for use among users. Multivitamins are the most frequently used supplements. Family members and TV advertisements were the main sources of information, in addition to limited advice from healthcare professionals. A minority of participants reported gastrointestinal side effects. Herbal medicine and dietary supplement use were more prevalent among patients with stage IV cancer and renal cell carcinoma (RCC). Discussion: This study revealed that the limited uptake of herbal medicines and dietary supplements alongside ICI treatment among cancer patients. The lack of adequate information from healthcare professionals poses potential risks to patients. Improved communication with patients, education regarding herbal medicine and dietary supplement use, potential interactions, and associated risks during ICI treatment are essential. Further research is needed to identify the specific needs of patients, anticipated benefits, and potential harms of herbal medicine and dietary supplement use, together with ICIs.