There are limited studies on the adequacy of prisoner diet and food practices, yet understanding these are important to inform food provision and assure duty of care for this group. The aim of this study was to assess the dietary intakes of prisoners to inform food and nutrition policy in this setting. This research used a cross-sectional design with convenience sampling in a 945-bed male high-secure prison. Multiple methods were used to assess food available at the group level, including verification of food portion, quality and practices. A pictorial tool supported the diet history method. Of 276 eligible prisoners, 120 dietary interviews were conducted and verified against prison records, with 106 deemed plausible. The results showed the planned food to be nutritionally adequate, with the exception of vitamin D for older males and long-chain fatty acids, with Na above upper limits. The Australian dietary targets for chronic disease risk were not achieved. High energy intakes were reported with median 13·8 (se 0·3) MJ. Probability estimates of inadequate intake varied with age groups: Mg 8 % (>30 years), 2·9 % (<30 years); Ca 6·0 % (>70 years), 1·5 % (<70 years); folate 3·5 %; Zn and I 2·7 %; and vitamin A 2·3 %. Nutrient intakes were greatly impacted by self-funded snacks. Results suggest the intakes to be nutritionally favourable when compared with males in the community. This study highlights the complexity of food provision in the prison environment and also poses questions for population-level dietary guidance in delivering appropriate nutrients within energy limits.