Antonio Piccolomini, O.S.B. (died 1459) was a Roman Catholic prelate who served as Archbishop of Siena (1459) and Bishop of Siena (1458–1459).^[1][2]

Antonio Piccolomini was the son of Andrea Piccolomini, of the lords of Modanella, in the same branch of the Piccolomini family as Pope Pius II.^[3]

He was ordained a priest in the Order of Saint Benedict.^[2] He was appointed abbot of the Camaldolese monastery of S. Vigilio in Siena, which was under the patronage of the Piccolomini family.^[4] He was succeeded by Francesco Todeschini-Piccolomini (Pope Pius III), who was named Abbot Commendatory in 1458.^[5]

On 18 September 1458, Antonio was appointed Bishop of Siena by the newly-elected Pope Pius II, who had just vacated the bishopric of Siena.^[6] Pius II granted him the privilege of being consecrated a bishop by any bishop he chose who was in communion with Rome.^[7] As bishop-elect, he took possession of his new diocese on 28 September 1458.^[8] On 16 April 1459, he was consecrated bishop, though the names of the consecrators remain unknown.^[2]

On 18 April 1459, at the urging and with the cooperation of Pius II, who lent Bishop Antonio the services of Agapito Cincio de' Rusticii, a Referendary and Auditor Causarum in the papal Court, there was issued a new Constitution for the diocese, consisting of fifty-six articles.^[9]

On 23 April 1459, Pope Pius II elevated the diocese of Siena, of which he had recently been the bishop, to the rank of Metropolitan Archbishop of Siena.^[10]

Antonio Piccolomini served as Archbishop of Siena a total of seventeen months, until his death on 8 November 1459.^[1][2] He had gone to Monte Amiata to the baths of S. Filippo, for health reasons, and died at the castle of Crevole, which belonged to the bishops of Siena.^[11] He was succeeded on 6 February 1460 by Francesco Todeschini-Piccolomini.^[12]