BACKGROUND: People in prison experience poorer mental and physical health compared to their peers in the general population. The causes are multi-dimensional ranging from lifestyle factors to poorer access to healthcare. Little is known about cancer in people in prison or how the cost of their care compares to the general population. METHODS: Data on people diagnosed with cancer while in English prisons were identified in National Cancer Registration dataset and linked to Hospital Episode Statistics (HES) for the years 2012-2017. General population matched patients were identified using a 1-5 ratio, based on age, gender, year of diagnosis, cancer type and disease stage. Outpatient and inpatient HES data up to six-months from diagnosis were costed using NHS Reference costs and inflated to 2017/2018 costs. FINDINGS: 879 prison and 4326 general population cancer diagnoses were identified in HES. The adjusted six-month cost of cancer care was significantly lower for people in prison (-£1216.95% confidence interval (CI) -1638 to -795), driven by fewer outpatient attendances. However, people diagnosed in prison had higher emergency care costs (£497.95% CI 375-619). Security escorts further increased the total cost of care. INTERPRETATION: Following a cancer diagnosis, people in English prisons have significantly lower planned care costs, but higher emergency care costs and an overall higher cost due to security escorts. Further work is required to identify ways of improving cancer care for people in prisons to ensure it is equivalent to that received by the general population. FUNDING: National Institute for Health and Social Care Research 16/52/53.