People with severe mental illness often have suboptimal physical health and experience a wide range of comorbidities, including cancer, that affect health outcomes. Cancer nurses may encounter patients with mental health issues such as depression or anxiety, but these conditions often arise as a reaction to a cancer diagnosis, whereas severe mental illness often predates a cancer diagnosis. The signs and symptoms of severe mental illness can make it harder for people to engage in their cancer care, which can present complex challenges for cancer nurses. This article explores cancer care in relation to severe mental illness and examines some important signs and symptoms and how they may affect cancer care. The article also considers ways in which cancer nurses can support person-centred cancer care for people with severe mental illness.