Giuseppe Cardinal Siri (20 May 1906 - 2 May 1989) was a senior cardinal of the Roman Catholic Church. As Archbishop of Genoa, he was one of the more conservative Catholic prelates of the Second Vatican Council. He had been raised to the cardinalate by Pope Pius XII.

It has been claimed that Siri was in fact elected to the papacy twice; in 1958 and 1963 (even announcing in the Papal Conclave that he wished to be known as Pope Gregory XVII) but that on both occasions when faced with threats that Catholics in the Eastern Bloc would face persecution on account of his fiercely anti-Soviet Union opinions in the event of his assuming the papacy, he declined the Papal Tiara, a not unknown occurrence. Given that the conduct of papal conclaves is strictly confidential and that any cardinal revealing the details would face instant excommunication, no documentary evidence has ever been proven to substantiate or disprove the widely claimed rumour.

Siri was a leading candidate for the papacy in both the August and October 1978 conclaves that followed the deaths of Pope Paul VI and Pope John Paul I respectively. Media reports suggested that Siri in fact topped the first count of votes in the August conclave but ultimately was beaten by Albino Cardinal Luciani, who became Pope John Paul I. Following Luciani's death in the papacy, Siri was the leading conservative candidate against Giovanni Benelli, the leading liberal candidate. Vatican writers suggested that the eventual winner, Karol Wojtyla, who became Pope John Paul II was chosen as a compromise candidate between the two.

Though championed by conservative Catholics following the widespread rumours that he had actually been elected to the papacy in 1958 and 1963 (when the eventual winners were Angelo Cardinal Roncalli (Pope John XXIII) and Giovanni Cardinal Montini (Pope Paul VI)) Siri remaining in full communion with the Catholic Church and refused to support any sedevacantist conservative catholic organisation. He died on 2nd May 1989.