Objective: To explore how caregivers are involved in making treatment decisions for older people living with dementia and a new diagnosis of cancer. Method: A systematic review of PubMed, CINAHL, PsycINFO, Web of Science, and Scopus databases was conducted. Studies recruiting formal or informal caregivers for older people with dementia and a diagnosis of cancer were considered for inclusion. Results: Of 1761 articles screened, 36 full texts were assessed for eligibility, and six were included in the review. This review has identified that health care professionals (HCPs) are often unaware of the coexistence or severity of dementia in cancer patients, and therefore fail to properly address care needs as a result. While caregivers are relied on to help make decisions, they have unmet information needs and feel excluded from decision-making. Conclusion: Treatment decision making in the context of older adults with dementia and a new diagnosis of cancer needs further research. This will help HCPs to understand their needs and improve the experience of decision making for both caregivers and the people that they care for.