Louis de Blois O.S.B. (October 1506 – 7 January 1566), also known by the Latinized name Ludovicus Blosius, was a Flemish Benedictine abbot, mystical theologian, and spiritual writer associated with the transmission of late medieval Christian mysticism into the spirituality of the Catholic Reformation. As abbot of Liessies Abbey, he became an influential figure in monastic reform in the Habsburg Netherlands and one of the most widely read devotional authors of the sixteenth and seventeenth centuries. His writings synthesized elements from the Rheno-Flemish mystical tradition, especially Johannes Tauler, Jan van Ruusbroec, Henry Suso, and Hendrik Herp, while remaining rooted in Benedictine asceticism, sacramental devotion, and obedience.

Modern historian Bernard McGinn describes Blosius as "the only major French mystic of the first half of the sixteenth century" and argues that histories of French mysticism often begin too abruptly in the 1590s, thereby overlooking earlier Catholic mystical responses to the Reformation. His best-known works include the Institutio spiritualis (Book of Spiritual Instruction), the Speculum spirituale, and the Consolatio pusillanimium (Comfort for the Faint-Hearted).

Louis de Blois was born in October 1506 at the château of Donstiennes near Thuin, then in the Prince-Bishopric of Liège. His father was Adrien de Blois, lord of Jumigny, from the family of the counts of Blois-Châtillon, and his mother was Catherine de Barbençon, lady of Donstiennes. Although partly of French noble ancestry, later scholars have often treated him as belonging culturally and spiritually to the Low Countries.

As a child he received a refined education before being sent as a page to the court at Ghent of the future emperor Charles V, with whom he maintained friendly relations throughout his life. Later biographical tradition records that a serious head injury and operation helped turn him away from courtly ambitions toward the religious life.

In 1520, at about fourteen years of age, he entered the Benedictine abbey of Liessies in Hainaut as a novice under Dom Jean Meurisse. Between 1524 and 1530 he studied arts and theology at the University of Louvain, where he pursued humanist studies in Latin, Greek, and Hebrew at the Collegium Trilingue and studied theology under Ruard Tapper and Jean Driedo. In 1527 the elderly abbot Gilles Gippus designated him coadjutor and successor. After Gippus's death on 2 March 1530, Louis returned to Liessies, where he was ordained priest on 11 November, celebrated his first Mass the following day, and was installed as abbot on 13 November.

Under Blosius, Liessies became an important centre of monastic reform, spiritual direction, and mystical theology. His reforming activity extended beyond his own monastery and influenced other Benedictine communities in the Low Countries. The reform initially encountered resistance within the monastery. During renewed warfare between Francis I of France and Charles V in 1537, many monks fled Liessies because of its frontier location. Blosius relocated to Ath with three monks committed to stricter observance and began a reform experiment there that soon attracted further followers. After the restoration of peace, Charles V ordered the community to return to Liessies in 1539, where Blosius implemented a moderated but lasting reform of Benedictine observance. In 1545 Pope Paul III approved his Statuta monastica, reflecting his role in Catholic monastic renewal during the Reformation era.

Blosius supported the establishment of the first Jesuit foundation at Louvain and encouraged the expansion of the Society of Jesus in northern Europe. He personally undertook the Spiritual Exercises and defended the Jesuits before civil authorities. He was also associated with the Cologne Carthusians, especially the circle involved in editing, translating, and disseminating medieval mystical texts.

Charles V repeatedly sought to advance Blosius to higher ecclesiastical office, including the abbacy of Saint-Martin at Tournai and the archbishopric of Cambrai, but Blosius declined both appointments in order to remain at Liessies. In addition to reforming the monastery, he enlarged the abbey library with manuscripts of saints' lives, martyrologies, and devotional texts, helping make Liessies one of the notable hagiographical collections of its age. Later tradition associated this library with the inspiration for Heribert Rosweyde's conception of the Acta Sanctorum project eventually continued by the Bollandists.