William Allen (1532 – 16 October 1594), also known as Guilielmus Alanus or Gulielmus Alanus, was an English Cardinal of the Roman Catholic Church. He was an ordained priest, but was never a bishop. His main role was setting up colleges to train English missionary priests with the mission of returning secretly to England to keep Roman Catholicism alive there. Allen assisted in the planning of the Spanish Armada's attempted invasion of England in 1588. It failed badly, but if it had succeeded he would probably have been made Archbishop of Canterbury and Lord Chancellor. The Douai-Rheims Bible, a complete translation into English from Latin, was printed under Allen's orders. His activities were part of the Counter Reformation, but they led to an intense response in England and in Ireland. He advised and recommended Pope Pius V to pronounce Elizabeth I deposed. After the Pope declared her excommunicated and deposed, Elizabeth intensified the persecution of her Roman Catholic religious opponents.

William Allen was born in 1532, at Rossall Hall in Rossall, Lancashire, England. He was the third son of John Allen by his marriage to Jane Lister. In 1547, at the age of fifteen, he entered Oriel College, Oxford, graduated Bachelor of Arts in 1550, and was elected a Fellow of his College. In 1554, he was promoted by seniority to Master of Arts, and two years later, in 1556, was made Principal and Proctor of Saint Mary's Hall.

Allen seems also to have been a canon at York Minster in or about 1558, indicating that he had most likely received tonsure, the initial step towards ordination that conferred clerical status. Upon the accession of Elizabeth I, he refused to take the Oath of Supremacy, but was allowed to remain at the University of Oxford until 1561.

His public opposition to the newly Protestantized Church of England forced him to leave the country, and in 1561, after resigning his benefices, he left England to seek refuge at Louvain and its University, where he joined many other academics from Oxford and Cambridge who had refused to take the Oath of Supremacy. There, he continued his theological studies and began to write apologetic, polemic, and controversialist treatises. In the following year, because of ill health, he was advised to return to his native Lancashire. He devoted himself to the re-conversion of his native land to the old faith. In particular, he worked to dissuade the Roman Catholic faithful from attending Anglican worship, an outward compromise of their faith and conscience that many made, to avoid ruin from fines, confiscations, and other disabilities.

During this period as a clandestine missionary in England, Allen formed the conviction that the people were not set against Rome by choice, but by force and by circumstances; and the majority were only too ready, in response to his sermons and ministrations, to return to Roman Catholicism. He was convinced that the Protestant hold over the Kingdom, favoured by the policies of Elizabeth, could only be temporary. When his presence was discovered by the Queen's agents, he fled from Lancashire and withdrew to Oxford, where he had many acquaintances.

After writing a treatise in defence of the power of the priest to remit sins, Allen was obliged to move to Norfolk, under the protection of the family of the Duke of Norfolk, but already in 1565 had once again to leave for the Continent. He was never to return. Travelling to the Low Countries, he was ordained as a priest shortly afterwards at Malines in Flanders and began to lecture in theology at the Benedictine College there.

William Allen was always convinced that the Protestant wave over the country, due to the action of Elizabeth, could only be temporary and that the whole future depended on there being a supply of trained clergy and controversialists ready to come into the country whenever Catholicism should be restored. In 1567, Allen went to Rome for the first time and conceived his plans for establishing a College where students from England and Wales could live together and finish their theological education. The idea subsequently developed into the establishment of a missionary college, or seminary, to supply England with priests as long as the schism with the See of Rome persisted. With the help of friends, and notably of the Benedictine abbots of the neighbouring monasteries, a beginning was made in a rented house at Douai on Michaelmas Day, 29 September 1568, which marked the inauguration of the English College, Douai.

Allen was to be joined by many English exiles, including Edmund Campion. Douai was thought suitable for Allen's new College because of the recent foundation there of the University of Douai by Pope Paul IV, under the patronage of King Philip II of Spain, to whose dominions Douai then belonged, and because the foundation had the active encouragement of Jean Vendeville, a law professor at the university, who had accompanied Allen on his journey back from Rome. On 31 January 1570, through the influence of Vendeville, Allen was given the post of Professor of Sacred Scripture.