ABSTRACT Objective: to understand the perceptions about the values of breastfeeding for nursing mothers under custody of the penitentiary system. Method: a qualitative and phenomenological research study based on the Schelerian values, with seven phenomenological interviews conducted with nursing mothers deprived of their freedom in a women's prison unit from Rio de Janeiro, Brazil. The interviews took place between November 2020 and May 2021. Content analysis for treatment of the results was performed after transcribing the testimonies. Results: two categories were identified for the vital value: The value of bonding as a breastfeeding expansion process; and The nursing mother as a safety and protection value for the infant. Through the perception of the nursing mothers deprived of their freedom, the vital value identified acts as protection for breastfeeding within the prison setting, strengthening the woman's and child's health. Conclusion: by unveiling the vital value, it is observed that it is related to the mother's and infant's everyday life in prison. However, the prison institution does not meet the real vital needs of imprisoned nursing mothers, with the consequent need to redirect the breastfeeding practice in the prison setting, transforming it into a legitimate environment that values the meaning of breastfeeding.