Martin Anton Delrio SJ (Latin: Martinus Antonius Delrio; Spanish: Martín Antonio del Río; French: Martin-Antoine del Rio; 17 May 1551 – 19 October 1608) was a Dutch Jesuit theologian. He studied at numerous institutions, receiving a master's degree in law from Salamanca in 1574. After a period of political service in the Spanish Netherlands, he became a Jesuit in 1580.

He studied or taught at Jesuit colleges across Catholic Europe, including Bordeaux, Douai, Graz, Mainz, Leuven, and Salamanca. He was the friend of the Flemish humanist Justus Lipsius, a relative of Michel de Montaigne, and an enemy of the Protestant scholar Joseph Scaliger. He was the author of a large number of books, including classical commentaries and works of biblical exegesis. He remains, however, best known for his six-volume Magical Investigations (1599–1600), a work on magic, superstition, and witchcraft.

Martin Delrio was born in Antwerp on 17 May 1551, Whit Sunday, to the Spanish merchant Antonio del Río (d. 17 February 1586) and his wife Eleonora López de Villanova (d. 21 April 1602). The Del Río family were part of a sizeable Spanish community in Antwerp, more than 200 merchants were active in Antwerp in 1540. Young Martin studied at a Latin school in nearby Lier and soon revealed himself as a childhood prodigy. He matriculated at the Old University of Leuven on 1 December 1563, at the age of 12.

There he studied under the humanist Cornelius Valerius and met a number of other young promising scholars, including Andreas Schott, Willem Canter, and Justus Lipsius. In middle age, their friendship would significantly change the course of both their lives. Delrio's first publication, an edition of the late Roman grammarian Gaius Iulius Solinus, was based on a manuscript borrowed from Lipsius and included suggested emendations by his tutor Valerius. Delrio also published an edition of Claudian. He was particularly proud of the edition of Senecan tragedy, published in 1576 but which he (falsely) claimed to have completed before his twentieth birthday.

His travels during his peregrinatio academica are difficult to follow. He can be placed at the University of Paris in 1567 and 1568. He also spent some time in Douai where he refused to share a bed with an unnamed famous man (cited by his 1609 Jesuit hagiography as proof of his chastity). In 1572 he matriculated at the University of Salamanca on 1 December 1572 and graduated two years later. At Salamanca he would see ‘the remnant of an evil gymnasium’ where Muslims had allegedly taught magic.

Martin Delrio was never destined for one of the religious orders. His family had originally destined him for a political career. His law degree from Salamanca was part of this requirement. The Spanish crown since the days of Catholic Monarchs had particularly valued such degrees. Equally valued were titles of nobility. A legacy left by Martin's grandfather was used to buy him the title of Lord of Aartselaar. On 7 September 1561, at the age of 10, Martin Delrio made his official entry, swearing an oath in which he promised to protect widows and orphans. The Delrio family also paid host to a number of prominent figures on visits to Antwerp, including Antoine Perrenot de Granvelle and Anne of Austria, Queen of Spain, Philip II's fourth wife.

With the onset of the Dutch Revolt members of the family found employment with the Council of Troubles, the repressive institution set up by the new governor-general Fernando Álvarez de Toledo, 3rd Duke of Alba. A cousin, Luís del Río, was one of the councillors. Martin's father Antonio later became treasurer-general of confiscations, in charge of seizing assets. Martin himself would after his return from Spain (sometime in 1576 or early 1577) follow suit. The new governor-general Don John of Austria first made peace and then broke it. Soon after, on 29 October 1577, Don John appointed Martin to the Council of Brabant. Credentials or qualifications were irrelevant; with the exception of one loyalist member who fled to Paris, none of the existing councillors had decided to follow Don John.

Despite Martin's appointment and rapid promotion (he would be made vice-chancellor of Brabant in July 1578), these were tragic years for the Delrio family. Luís was arrested by the rebels, later released but soon died. Martin's father Antonio evaded capture and died a penniless exile in Lisbon. Martin's career also faltered. After the death of Don John, the new governor-general Alexander Farnese, Duke of Parma changed course, embarking on a campaign reconciliation. Little attention was paid to those whose loyalty could be taken for granted. Martin seems to have lost his position as vice-chancellor. Another victim seems to have been the memoirs which Delrio composed in honour of his patron Don John of Austria and in ostensible imitation of Caesar's De Bello Gallico. These remained unpublished until the late nineteenth century.