Popes with the shortest reigns in history

Urban VII (September 15–September 27, 1590): reigned for 13 calendar days, died before consecration.

Boniface VI (April, 896): reigned for 16 calendar days.

Celestine IV (October 25–November 10, 1241): reigned for 17 calendar days, died before consecration.

Theodore II (December, 897): reigned for 20 calendar days.

Sisinnius (January 15–February 4, 708): reigned for 21 calendar days.

Marcellus II (April 9–May 1, 1555): reigned for 22 calendar days.

Damasus II (July 17–August 9, 1048): reigned for 24 calendar days.

Pius III (September 22–October 18, 1503): reigned for 27 calendar days.

Leo XI (April 1–April 27, 1605): reigned for 27 calendar days.

Benedict V (May 22–June 23, 964): reigned for 33 calendar days.

John Paul I (August 26–September 28, 1978): reigned for 33 calendar days.