The cross-sectional study assessed knowledge, attitudes, and preventive practices toward COVID-19 disease of incarcerated people. A total of 685 subjects were surveyed. 94% were aware that respiratory droplets are involved in the transmission of COVID-19, and 77.2% that patients with chronic conditions are at risk of a more severe disease. Overall, 92.7% of respondents considered COVID-19 a more severe disease compared to influenza, and 85.4% believed that COVID-19 could cause serious consequences in their institution. Only 22.6% were self-confident about their ability to protect themselves from SARS-CoV-2 infection. This attitude was significantly higher in those who were involved in working activities in the institution, who did not report at least one common symptom compatible with COVID-19 in the previous 3 months, who did not show generalized anxiety symptoms, and did not need additional information. 63.9% of incarcerated people expressed willingness to receive COVID-19 vaccination. Older subjects, who knew that a COVID-19 vaccination is available, believed that COVID-19 is more serious than influenza, and were self-confident about their ability to protect themselves from SARS-CoV-2 infection, were significantly more willing to undergo COVID-19 vaccination. Public health response to COVID-19 in prisons should address vaccine hesitancy to increase vaccine confidence among incarcerated people.