This study aimed to describe the knowledge and practice of insulin self-injection among patients with diabetes mellitus before discharge at Dai Tu District General Hospital in 2025. A cross-sectional descriptive study was conducted on 50 patients treated at the Department of Internal Medicine from January to September 2025 who were indicated to continue insulin therapy at home. Data were collected through direct interviews, observation using a questionnaire and processed with Microsoft Excel 2016. The results showed that females accounted for 52.0%, patients aged 60 years and older for 56.0%, those with a disease duration of more than 5 years for 78.0%, and patients with comorbidities for 84.0%. Regarding knowledge, 86.0% knew that hypoglycemia was a common adverse effect of insulin, and 92.0% knew how to manage hypoglycemia by drinking sugar water or eating sweets; however, only 26.0% knew that capillary blood glucose testing was necessary. In addition, 92.0% knew how to store unused insulin in the refrigerator. Regarding practice, 86.0% knew that injection sites should be changed, but only 8.0% knew that the buttock was a possible injection site. Patients still had limitations in antiseptic procedures, air-bubble checking, and keeping the needle under the skin for a sufficient time. Counseling and assessment of insulin self-injection skills of the patients before discharge should be strengthened.