Pope Nicholas V (Latin: Nicolaus V; Italian: Niccolò V; 15 November 1397 – 24 March 1455), born Tommaso Parentucelli, was head of the Catholic Church and ruler of the Papal States from 6 March 1447 until his death in March 1455. Pope Eugene IV made him a cardinal in 1446 after successful trips to Italy and Germany, and when Eugene died the next year, Parentucelli was elected in his place. He took his name Nicholas in memory of his obligations to Niccolò Albergati. He remains the most recent pope to take the pontifical name "Nicholas".

The pontificate of Nicholas saw the fall of Constantinople to the Ottoman Turks and the end of the Hundred Years' War. He responded by calling a crusade against the Ottomans, which never materialized. By the Concordat of Vienna he secured the recognition of papal rights over bishoprics and benefices. He also brought about the submission of the last of the antipopes, Felix V, and the dissolution of the Synod of Basel. A key figure in the Roman Renaissance, Nicholas sought to make Rome the home of literature and art. He strengthened fortifications, restored aqueducts, and rebuilt many churches. He ordered design plans for what would eventually be the Basilica of St. Peter.

Tommaso Parentucelli was born in Sarzana, an important town in Lunigiana, to the physician Bartolomeo Parentucelli and wife Andreola Bosi of Fivizzano. The Lunigiana region had long been fought over by competing Tuscan, Ligurian and Milanese forces. Three years before Parentucelli's birth, the town was captured from the Florentines by the Genoese Republic. His father died while he was young. Parentucelli later became a tutor, in Florence, to the families of the Strozzi and Albizzi, where he met the leading humanist scholars.

Parentucelli studied at Bologna and Florence, gaining a degree in theology in 1422. Bishop Niccolò Albergati was so awestruck with his capabilities that he took him into his service and gave him the chance to pursue his studies further by sending him on a tour through Germany, France and England. He was able to collect books, for which he had an intellectual's passion, wherever he went. Some of them survive with his marginal annotations.

Parentucelli attended the Council of Florence and in 1444, when his patron died, he was appointed Bishop of Bologna in his place. Civic disorders at Bologna were prolonged, so Pope Eugene IV soon named him as one of the legates sent to Frankfurt. He was to assist in negotiating an understanding between the Papal States and the Holy Roman Empire, regarding undercutting or at least containing the reforming decrees of the Council of Basel (1431–1439).

Parentucelli's successful diplomacy gained him the reward, on his return to Rome, of the title Cardinal-Priest of Santa Susanna in December 1446. At the papal conclave of 1447 he was elected pope in succession to Eugene IV on 6 March. He took the name Nicholas in honour of his early benefactor, Niccolò Albergati.

In only eight years, his pontificate delivered important achievements in the political, scientific, and literary history of the world. Politically, he needed to repair relationships which had broken down in the pontificate of Eugene IV. He called the congress which produced the Treaty of Lodi, secured peace with Charles VII of France, and concluded the Concordat of Vienna or Aschaffenburg (17 February 1448) with the German King, Frederick III, by which the decrees of the Council of Basel against papal annates and reservations were abrogated so far as Germany was concerned. In the following year he secured a still greater tactical triumph with the resignation of the Antipope Felix V on 7 April and his own recognition by the rump of the Council of Basel that assembled at Lausanne.

In 1450, Nicholas held a Jubilee at Rome, and the offerings of the numerous pilgrims who thronged to Rome gave him the means of furthering the cause of culture in Italy, which he had so much at heart. In March 1452 he crowned Frederick III as Holy Roman Emperor in St. Peter's, in what was the last imperial coronation held in Rome.