Joan was the daughter of Isabella of Coimbra and Afonso V of Portugal. She was born on 6 February 1452, and she was for a short time the heiress presumptive between the death of her older brother John and the birth of her younger brother, the future John II. She was even given the title Princess of Portugal at this time despite the fact that she was only the heiress presumptive. Usually, she would have been titled as Infanta. Even after the birth of her younger brother she was referred to as Princess. In 1471 Joan acted as regent in her father’s absence.
Joan was greatly drawn to the religious life, but due to her close proximity to the throne, her father forbade her from becoming a nun. She was finally able to join the convent after her brother had a child in 1475, but offers of marriage continued to come. A serious candidate was the recently widowed Richard III of England. In a double marriage alliance, Joan would marry Richard, and Richard’s niece Elizabeth of York would marry the future Manuel I of Portugal. Joan was a descendant of John of Gaunt, through his daughter Philippa of Lancaster. Richard would die in battle a short while later, and neither marriage took place. Joan supposedly had a dream that Richard would die in battle. Joan would remain unmarried for the rest of her life.
Joan died on 12 May 1490 at the age of 38 in the convent at Aveiro. Her brother died without surviving issue five years later and had she outlived him she would have become Queen Regnant. Instead, her brother was succeeded by their first cousin Manuel. Though Joan was beatified in 1693, she was never canonised so she cannot be considered a saint. However, she is commonly referred to as Princess Saint Joan. She is buried in the convent in Aveiro. Her tomb survives to this day, and you can visit the convent.