Pietro Ottoboni (2 July 1667 – 29 February 1740) was an Italian cardinal and grandnephew of Pope Alexander VIII (who was also born Pietro Ottoboni). He is remembered especially as a great patron of music and art. Ottoboni was the last person to hold the curial office of Cardinal-nephew, which was abolished by Alexander VIII's successor, Pope Innocent XII, in 1692. Ottoboni "loved pomp, prodigality and sensual pleasure, but was in the same time kind, ready to serve and charitable".
Pietro was born in Venice to the noble Ottoboni family, whose most prominent member had been his granduncle Pope Alexander VIII (1689–1691). The family had bought their way into Venetian nobility in the 17th century.
He received the clerical tonsure and the minor orders on 20 October 1689 and was created cardinal deacon in the consistory of 7 November 1689, receiving the red hat on 14 November. He was superintendent general of the affairs of the Apostolic See, and governor of the cities of Fermo and Tivoli, as well as of the territory of Capranica.
He held the office of vice-chancellor of the Holy Roman Church from 14 November 1689 until 29 February 1740. He was cardinal-bishop of Sabina 1725, cardinal-bishop of Frascati, 1730, Cardinal-bishop of Porto and Santa Rufina, 1734, and vice-dean then dean of the Sacred College of Cardinals (3 September 1738). Ottoboni was also Archpriest of the patriarchal Liberian Basilica, secretary of the Roman Inquisition, Archpriest of the patriarchal Lateran Basilica (from 1730) and Grand prior of Ireland.