Currently, various immunotherapeutic treatments are revolutionizing therapies that treat solid neoplasms. For these treatments, within immunotherapy, immune checkpoint inhibitors (ICIs) are the most widely used drugs. Diverse studies have shown the influence of diet and probiotics on the response to ICIs and consequently on the survival rates associated with different neoplasms. The use of various antibiotics, probiotics, and prebiotics has been associated with changes in the gut microbiota, and this, in turn, with resistance to immunotherapy. Together with the above, a lower intake of red meat and greater consumption of a Mediterranean, vegetarian, or vegan diet have led to a new way of understanding the mechanisms of resistance to ICIs. Omega-3 and polyphenol supplements are also powerful regulators of the microbiome whose influence on the immune system. Therefore, this review covers the influence of diet and probiotics on the response to immunotherapy in patients who have solid tumours.