Jacopo Piccolomini-Ammannati, or Giacomo Piccolomini (1422 – 10 September 1479) was an Italian Renaissance cardinal and humanist.
He was born at Pescia near Lucca. He was related to the Piccolomini of Siena. His literary and theological education he acquired in Florence. Under Pope Nicholas V he went to Rome, where, for a while, he lived in extreme penury.
In 1450 he became private secretary to Cardinal Domenico Capranica; later Pope Callixtus III appointed him Secretary of Briefs. He was retained in this office by Pope Pius II, who also made him a member of the pontifical household, on which occasion he assumed the family name of Piccolomini. In 1460 he was made Bishop of Pavia by Pius II, and throughout the pontificate of the latter was his most trusted confidant and adviser. He exhibited paternal solicitude in the government of his diocese, and during his prolonged absences entrusted its affairs to able vicars, with whom he remained in constant touch.
On 18 December, 1461, he was made cardinal, and was commonly known as the Cardinal of Pavia. He accompanied Pius II to Ancona, and attended him in his last illness. In the subsequent conclave he favoured the election of Pope Paul II, whose displeasure he afterward incurred by insisting on the full observance of the ante-election capitulations that the pope had signed. The imprisonment of his private secretary by Paul II on a charge of complicity in the conspiracy of the Accademici offended Piccolomini still more, and his open defence of the secretary aggravated the pope's ill-will. The disfavour in which he was held by Paul II did not exempt his episcopal revenues from sequestration by the Duke of Milan, Galeazzo Maria. It was due to his insistence that Paul II took energetic measures against king George Podiebrad of Bohemia. Pope Sixtus IV was scarcely more favourable towards Piccolomini than Paul II.
He was the friend of students and scholars, and protected Jacopo de Volterra. Ammannati is one of the most sympathetic personalities of the Italian Renaissance. He enjoyed the friendship of noted prelates and humanists, such as Cardinal Carvajal and Cardinal Roverella. Cardinal Bessarion Pastor ("Geschichte der Päpste", II, 731), praises his executive ability and readiness, charity and zeal.
He wrote a continuation in seven books of the Pius II Commentarii. His style is elegant, but he is not always impartial, especially apropos of Pope Paul II or Sixtus IV. His Commentaries, nevertheless, remain an important source for contemporary history and his valuable letters have been collected and published.