Domenico Tardini (29 February 1888 – 30 July 1961) was a longtime aide to Pope Pius XII in the Secretariat of State. Pope John XXIII named him Cardinal Secretary of State and, in this position the most prominent member of the Roman Curia in Vatican City.

He attended the Angelo Braschi School and entered the Pontifical Roman Seminary in 1903 from which he graduated with honours in philosophy and theology. 21 September 1912 he was ordained a priest. He accepted a call to teach liturgy and theology at the Roman Seminary and the Collegio Urbano of the Propaganda Fide. In 1923, he was nominated by Pope Pius XI to be general assistant of the Catholic Action movement. In 1925, the Pope nominated him to a second organisation, Società della Gioventù Cattolica Italiana. From 1921 onward, he worked as well in the Congregation of Ordinary Ecclesiastical Affairs where he was named Sustituto in 1929 and Secretary in 1937. With Giovanni Battista Montini, later Pope Paul VI, he was the main assistant to Cardinal Secretary of State Eugenio Pacelli, later Pope Pius XII, until 1939.

After his election as Pope Pius XII, Pacelli appointed Luigi Maglione as his successor as Cardinal Secretary of State. Maglione did not exercise the influence of his predecessor, who as Pope continued his close relation with Monsignori Giovanni Battista Montini and Tardini. After the death of Maglione in 1944, Pius left the position vacant and named Tardini head of its foreign section and Montini head of the internal section. Tardini and Montini continued serving there until 1952, when Pius XII decided to elevate both of them into the College of Cardinals an honour which both turned down. When Tardini thanked him for not appointing him, Pius XII replied with a smile:

In November 1952, he was named Pro-Secretary of the State for Extraordinary Ecclesiastical Affairs by Pope Pius XII, essentially co-serving as functional Secretary of State with Giovanni Battista Montini, who became Pro-Secretary of State for Ordinary Ecclesiastical Affairs. In addition they were granted the privilege to wear Episcopal Insignia. Tardini continued in that position until the death of Pius XII.

Tardini loved children and "adopted" the orphans of Villa Nazareth, for whom he organised recognition and assistance. Televised audiences with Pope Pius XII, and visits of Pope John XXIII and high-ranking foreign dignitaries, all arranged by Tardini, facilitated fundraising for the needy children.

After the death of Pope Pius XII on 9 October 1958, Tardini, though not a cardinal and not even a bishop, was prominently mentioned as a possible successor because of his familiarity with and closeness to Pius. Instead, following the conclave, Tardini was named Secretary of State by the newly-elected Pope John XXIII, filling the previous vacancy. Having declined being made Cardinal by Pius XII in 1953, he accepted the red hat in the consistory of 15 December 1958 with the title of Cardinal-Deacon of S. Apollinare alle Terme Neroniane-Alessandrine. He was ordained as a titular archbishop later that month.

Tardini did not seek the honour of the appointment, finally, of Cardinal Secretary of State. On the very night of the election of John XXII or as the first papal appointment next morning (biographers differ on this) Tardini met with the new Pope, who asked him to become his Secretary of State. He went reluctantly:

He did not give me any choice. I told the Holy Father, that I would not serve under him, because new policies would need new people. I reminded him that I frequently disagreed with him in the past. I reminded him that I was tired and worn out and that my health was getting worse. I told him about my long cherished ambition of at last giving myself entirely to the orphan boys of Villa Nazareth. It made no difference. The Pope listened to me with kindness and interest but to every point he replied, I understand but I want you to be my Secretary of State. Finally I knelt down and offered him my obedience.