The papal conclave of March–April 1605 was convened on the death of Pope Clement VIII and ended with the election of Alessandro Ottaviano de' Medici as Pope Leo XI on 1 April 1605. It was the first of two papal conclaves in 1605, with Leo dying on 27 April 1605, twenty-six days after he was elected, and the conclave to elect his successor being held in May. The conclave saw conflict regarding whether Cesare Baronius should be elected pope, and Philip III of Spain, the Spanish king, excluded both Baronius and the eventually successful candidate, Medici. Philip's exclusion of Medici was announced by Cardinal Ávila after his election to the papacy, and the other cardinals did not view it as valid since Medici had already been elected pope.

Nicholas II had reserved the right to elect the pope to the cardinal bishops, priests, and deacons of Rome in 1059. The cardinal bishops were the highest rank, being the bishops of the ancient suburbicarian dioceses, the priests ranked next, who served as the titular head of historically important churches in Rome, and last ranked the cardinal deacons, who were nominally assigned one of the ancient diaconia where traditionally deacons had administered the temporal property of the Church of Rome. Cardinals were required to have been ordained at least to the rank of their order within the College of Cardinals, but could also be ordained to a higher order as well.

In 1586, Pope Sixtus V mandated that the maximum number of cardinals would be seventy. Of these, the College of Cardinals had sixty-nine total members at the time of Clement VIII's death, but only sixty were present for the first conclave of 1605 when it opened, and sixty-one total electors were present for the election of Leo XI. The electors present had been created by six different popes: Pius IV, Gregory XIII, Sixtus V, Gregory XIV, Innocent IX, and Clement VIII. Of these, Clement's creations were the most numerous, having created thirty-eight of the cardinal electors. Innocent IX had created one of the conclave's electors, Gregory XIV had created five, Sixtus V had created eleven, Gregory XIII had created four, and Pius IV had created one.

Pietro Aldobrandini, the cardinal-nephew of Clement VIII, was the elector who controlled the largest number of votes with twenty-two of Clement's thirty-eight creations following his instructions. Alessandro Peretti di Montalto, the nephew of Sixtus V, controlled eight votes. Thirteen of the cardinal electors were loyal to the Spanish monarchy, and these electors and the faction loyal to Montalto were aligned. In addition to these groups, eight of the electors formed a faction that were loyal to the French crown.

List of cardinal electors

Name	Rank	Created cardinal by	Nationality	Sources
Tolomeo Gallio	Bishop	Pius IV	Italian
Alessandro Ottaviano de' Medici	Bishop	Gregory XIII	Italian
François de Joyeuse	Bishop	Gregory XIII	French
Domenico Pinelli	Bishop	Sixtus V	Italian
Girolamo Bernerio	Bishop	Sixtus V	Italian
Agostino Valier	Priest	Gregory XIII	Italian
Antonio Maria Gallo	Priest	Sixtus V	Italian
Antonmaria Sauli	Priest	Sixtus V	Italian
Benedetto Giustiniani	Priest	Sixtus V	Italian
Giovanni Evangelista Pallotta	Priest	Sixtus V	Italian
Federico Borromeo	Priest	Sixtus V	Italian
Francesco Maria del Monte	Priest	Sixtus V	Italian
Gregorio Petrocchini	Priest	Sixtus V	Italian
Mariano Pierbenedetti	Priest	Sixtus V	Italian
Paolo Emilio Sfondrati	Priest	Gregory XIV	Italian
Ottavio Paravicini	Priest	Gregory XIV	Italian
Ottavio Acquaviva d'Aragona	Priest	Gregory XIV	Italian
Flaminio Piatti	Priest	Gregory XIV	Italian
Giovanni Antonio Facchinetti de Nuce	Priest	Innocent IX	Italian
Pietro Aldobrandini	Priest	Clement VIII	Italian
Francesco Maria Tarugi	Priest	Clement VIII	Italian
Ottavio Bandini	Priest	Clement VIII	Italian
Anne d'Escars de Givry	Priest	Clement VIII	French
Giovanni Francesco Biandrate di San Giorgio Aldobrandini	Priest	Clement VIII	Italian
Camillo Borghese	Priest	Clement VIII	Italian
Caesar Baronius	Priest	Clement VIII	Italian
Lorenzo Bianchetti	Priest	Clement VIII	Italian
Francisco de Ávila	Priest	Clement VIII	Spanish
Francesco Mantica	Priest	Clement VIII	Italian
Pompeio Arrigoni	Priest	Clement VIII	Italian
Bonifazio Bevilacqua Aldobrandini	Priest	Clement VIII	Italian
Alfonso Visconti	Priest	Clement VIII	Italian
Domenico Toschi	Priest	Clement VIII	Italian
Girolamo Agucchi	Priest	Clement VIII	Italian
Paolo Emilio Zacchia	Priest	Clement VIII	Italian
Franz von Dietrichstein	Priest	Clement VIII	German
Robert Bellarmine	Priest	Clement VIII	Italian
François de Sourdis	Priest	Clement VIII	French
Séraphin Olivier-Razali	Priest	Clement VIII	French
Filippo Spinelli	Priest	Clement VIII	Italian
Carlo Conti	Priest	Clement VIII	Italian
Carlo Gaudenzio Madruzzo	Priest	Clement VIII	German
Jacques Davy Duperron	Priest	Clement VIII	French
Innocenzo del Bufalo-Cancellieri	Priest	Clement VIII	Italian
Giovanni Delfino	Priest	Clement VIII	Italian
Giacomo Sannesio	Priest	Clement VIII	Italian
Girolamo Pamphili	Priest	Clement VIII	Italian
Ferdinando Taverna	Priest	Clement VIII	Italian
Anselmo Marzato	Priest	Clement VIII	Italian
Erminio Valenti	Priest	Clement VIII	Italian
Francesco Sforza	Deacon	Gregory XIII	Italian
Alessandro Peretti di Montalto	Deacon	Sixtus V	Italian
Odoardo Farnese	Deacon	Gregory XIV	Italian
Cinzio Passeri Aldobrandini	Deacon	Clement VIII	Italian
Bartolomeo Cesi	Deacon	Clement VIII	Italian
Andrea Baroni Peretti Montalto	Deacon	Clement VIII	Italian
Alessandro d'Este	Deacon	Clement VIII	Italian
Giovanni Battista Deti	Deacon	Clement VIII	Italian
Silvestro Aldobrandini	Deacon	Clement VIII	Italian
Giovanni Andrea Doria	Deacon	Clement VIII	Italian
Carlo Emanuele Pio di Savoia	Deacon	Clement VIII	Italian