Cancer remains a leading cause of mortality worldwide, and conventional therapies such as chemotherapy, radiation, and surgery often come with severe side effects and limited efficacy in advanced stages. Phytochemicals, bioactive compounds derived from plants, have emerged as promising adjuncts in cancer therapy due to their ability to modulate signaling pathways, induce apoptosis, inhibit angiogenesis, and enhance the efficacy of conventional treatments while reducing toxicity. This review explores the role of phytochemicals such as curcumin, resveratrol, epigallocatechin gallate (EGCG), sulforaphane, and lycopene in cancer prevention and treatment. We evaluate preclinical and clinical evidence supporting their use, mechanisms of action, and potential synergies with standard therapies. The findings suggest that phytochemicals can improve therapeutic outcomes, mitigate adverse effects, and serve as cost-effective complementary agents in oncology.