Ferdinand the Holy Prince (Portuguese pronunciation: [fɨɾˈnɐ̃du]; Portuguese: Fernando o Infante Santo; 29 September 1402 – 5 June 1443), sometimes called the "Saint Prince" or the "Constant Prince", was an infante of the Kingdom of Portugal. He was the youngest of the "Illustrious Generation" of 15th-century Portuguese princes of the House of Aviz, and served as lay administrator of the Knightly Order of Aviz.

In 1437, Ferdinand participated in the disastrous Siege of Tangier led by his older brother Henry the Navigator. In the aftermath, Ferdinand was handed over to the Marinid rulers of Morocco as a hostage for the surrender of Ceuta in accordance with the terms of a treaty negotiated between the rulers of Portugal and Morocco by Henry. At first, Ferdinand was held in relative comfort as a noble hostage in Asilah, but when it became apparent that the Portuguese authorities had no intention of giving up Ceuta, Ferdinand's status was downgraded; he was transferred to a prison in Fez, where he was subjected to much harsher incarceration conditions by his jailers. Negotiations for his release continued intermittently for years, but they came to naught, and Ferdinand eventually died in captivity in Fez on 5 June 1443.

A popular cult quickly developed in Portugal around the figure of "the Holy Prince" (O Infante Santo), strongly encouraged by the House of Aviz. Ferdinand remains a "popular saint" by Portuguese tradition, neither beatified nor canonized by the Catholic Church.

Ferdinand was the sixth surviving child and youngest son of King John I of Portugal and his wife Philippa of Lancaster. Ferdinand and his brothers Edward of Portugal, Peter of Coimbra, Henry the Navigator and John of Reguengos, plus sister Isabella of Burgundy and half-brother Afonso of Barcelos, constitute what Portuguese historians have traditionally labelled the 'illustrious generation' (Ínclita Geração).

Ferdinand was born in Santarém on 29 September 1402, the feast day of St. Michael, a saint to whom he would remain affectionately attached. He had a complicated birth and would remain a sickly child throughout much of his youth. Relatively sheltered because of his illnesses, Ferdinand had a quiet and very pious upbringing, a favorite of his English mother, from whom he acquired a preference for the Sarum Rite of Salisbury in the religious liturgy of masses he attended.

Ferdinand was too young to participate in the 1415 Conquest of Ceuta led by his father, John I, in which his older brothers distinguished themselves and were knighted. As the youngest of many sons, Ferdinand did not obtain a substantial endowment from his father, only the Lordship of Salvaterra de Magos and a lifetime grant of Atouguia in 1429.

In 1434, after the death of his father John I and the administrator João Rodrigues de Sequeira, Ferdinand was appointed lay administrator of the Knightly Order of Aviz by his brother King Edward of Portugal. Ferdinand was also offered the titular office of cardinal by Pope Eugene IV, but turned it down. Despite his piety, Ferdinand had no intention of pursuing a clerical career.

In 1436, dissatisfied with his meager domains, Ferdinand asked his brother King Edward for permission to go abroad to seek his fortune in the service of a foreign king (reportedly, Henry VI of England). Ferdinand's request prompted the reluctant Edward to endorse a plan, long promoted by his brother Henry the Navigator, to launch a new Portuguese campaign of conquest against Marinid Morocco. As a bachelor, Ferdinand made out a will naming Edward's second son, the Infante Ferdinand (future Duke of Viseu) as his heir before departing.