Background: Diabetes mellitus (DM) is a growing non-communicable disease crisis in Nigeria, with rising prevalence among older adults and escalating complications burden on health systems. Nutritional knowledge, dietary self-care behaviour, and nutritional status are critical, interlinked determinants of glycaemic control, complication prevention, and quality of life for people living with DM. Despite the critical role of these factors, systematic assessment among hospitalised DM patients in South-Western Nigeria remains limited. Methods: We conducted a descriptive cross-sectional study among 161 inpatients with diabetes mellitus at Ekiti State University Teaching Hospital (EKSUTH), Ado Ekiti, using purposive and multistage sampling. A semi-structured interviewer-administered questionnaire assessed socio-demographic characteristics, nutritional knowledge (24 items), dietary self-care behaviour, and anthropometric measurements (BMI and WHR). We analysed data using SPSS version 25 with descriptive statistics and multiple linear regression. Results: Respondents were predominantly aged 61 and above (44.7%), female (54.0%), married (85.7%), and Yoruba (95.7%). While 85.7% correctly identified that untreated diabetes raises blood sugar, only 9.9% correctly refuted the myth that kidneys produce insulin, and 56.5% held the misconception that diabetic diets consist of special foods. Only 26.7–27.3% of respondents consistently adhered to key dietary self-care behaviours. Anthropometric assessment showed that only 9.3% had simultaneously healthy BMI and WHR; the remainder trended toward obesity or underweight. Regression analysis identified three significant independent predictors of nutritional status: belief that insulin reactions are caused by too much food (β=−0.187, p=0.025), belief that cuts heal more slowly in people with diabetes (β=−0.165, p=0.045), and belief that a diabetic diet consists of special foods (β=0.191, p=0.022). Conclusion: Critical nutritional knowledge gaps, low dietary self-care adherence, and widespread nutritional risk among DM inpatients at EKSUTH underscore the urgent need for structured, culturally adapted nutrition education programmes, individualised meal planning, and continuous professional development for healthcare providers managing diabetic patients in Ekiti State.