Worldwide, the incidences of cancer are rising. Various environmental and genetic factors predispose individuals to cancer. Many of these factors contribute to the upregulation of pro-survival pathways, downregulation of tumor suppressors, and chronic inflammation. The ratio of omega-6/omega-3 (ω-6/ω-3) polyunsaturated fatty acids (PUFAs) is a crucial factor in the initiation and progression of cancer. The low ω-6/ω-3 PUFA ratio has been demonstrated to be beneficial in managing the hallmarks of cancer cells. Enormous data from cancer cell lines and in vivo cancer models have provided insights into the mechanisms responsible for the anticancer effects of ω-3 PUFAs. We have discussed recent trends and anticancer effects of ω-3 PUFAs on immune checkpoint inhibitors and immunotherapy. We have also discussed major potential mechanisms for the beneficial effects of ω-3 PUFAs as demonstrated by the preclinical in vitro cancer cell line models and in vivo models.