Giuseppe Siri (20 May 1906 – 2 May 1989) was an Italian Cardinal of the Catholic Church who served as Archbishop of Genoa from 1946 to 1987, and was elevated to the rank of cardinal in 1953 by Pope Pius XII. He was a close friend of Pope Pius XII. He was considered a likely candidate to succeed Pius XII, John XXIII, Paul VI, and John Paul I.
Siri was noted for his strongly conservative views. The "Siri thesis" argues that Siri was elected pope in the 1958 papal conclave, taking the name Pope Gregory XVII, but that this was suppressed, with the then installed Pope John XXIII starting the liberal Second Vatican Council. Siri himself did not explicitly state support for the theory, but has been claimed to have given veiled support. The theory is held by some traditionalist Catholics and sedevacantists.
It may also be claimed that similarly occurred during the 1963 conclave that resulted in Pope Paul VI.