Hortus conclusus is a Latin term, meaning literally "enclosed garden". Both words in hortus conclusus refer linguistically to enclosure. It describes a type of garden that was enclosed as a practical concern, a major theme in the history of gardening, as walled gardens used to be more commonplace. The garden room is a similar feature, usually less fully enclosed.

Having roots in the Song of Songs in the Hebrew scriptures, the term hortus conclusus has importantly been applied as an emblematic attribute and a title of the Virgin Mary in Medieval and Renaissance poetry and art, first appearing in paintings and manuscript illuminations about 1330.

The term hortus conclusus is derived from the Vulgate Bible's Canticle of Canticles (also called the Song of Songs or Song of Solomon) 4:12, in Latin: "Hortus conclusus soror mea, sponsa, hortus conclusus, fons signatus" ("A garden enclosed is my sister, my spouse; a garden enclosed, a fountain sealed up.") This provided the shared linguistic culture of Christendom, expressed in homilies expounding the Song of Songs as allegory where the image of King Solomon's nuptial song to his bride was reinterpreted as the love and union between Christ and the Church, the mystical marriage with the Church as the Bride of Christ.

The verse "Thou art all fair, my love; there is no spot in thee" (4.7) from the Song was also regarded as a scriptural confirmation of the developing and still controversial doctrine of Mary's Immaculate Conception – being born without Original Sin ("macula" is Latin for spot).

Christian tradition states that Jesus Christ was conceived to Mary miraculously and without disrupting her virginity by the Holy Spirit, the third person of the Holy Trinity. As such, Mary in late medieval and Renaissance art, illustrating the long-held doctrine of the perpetual virginity of Mary, as well as the Immaculate Conception, was shown in or near a walled garden or yard. This was a representation of her "closed off" womb, which was to remain untouched, and also of her being protected, as by a wall, from sin. In the Grimani Breviary, scrolling labels identify the emblematic objects betokening the Immaculate Conception: the enclosed garden (hortus conclusus), the tall cedar (cedrus exalta), the well of living waters (puteus aquarum viventium), the olive tree (oliva speciosa), the fountain in the garden (fons hortorum), the rosebush (plantatio rosae). Not all actual medieval horti conclusi even strove to include all these details, the olive tree in particular being insufficiently hardy for northern European gardens.

The enclosed garden is recognizable in Fra Angelico's Annunciation (illustration at above left), dating from 1430-32.

Two pilgrimage sites are dedicated to Mary of the Enclosed Garden in the Dutch-Flemish cultural area. One is the statue at the hermitage-chapel in Warfhuizen: "Our Lady of the Enclosed Garden". The second, Onze Lieve Vrouw van Tuine (literally "Our Lady of the Garden"), is venerated at the cathedral of Ypres.

All gardens are by definition enclosed or bounded spaces, but the enclosure may be somewhat open and consist only of columns, low hedges or fences. An actual walled garden, literally surrounded by a wall, is a subset of gardens. The meaning of hortus conclusus suggests a more private style of garden.