The Hereford Mappa Mundi is a medieval map of the known world (Latin: mappa mundi), of a form deriving from the T and O pattern, dating from c. 1300. Archeological scholars believe the map to have originated from eastern England in either Yorkshire or Lincolnshire before it was transported westward to the Hereford Cathedral in Herefordshire where it has remained ever since. It is displayed at Hereford Cathedral in Hereford, England. It is the largest medieval map still known to exist. A larger mappa mundi, the Ebstorf map, was destroyed by Allied bombing in 1943, though photographs of it survive.

The Hereford Mappa Mundi hung, with little regard, for many years on a wall of a choir aisle in the cathedral. During the troubled times of the Commonwealth the map had been laid beneath the floor of Bishop Audley's Chantry, where it remained secreted for some time. In 1855 it was cleaned and repaired at the British Museum. During the Second World War, for safety reasons, the mappa mundi and other valuable manuscripts from Hereford Cathedral Library were kept elsewhere and returned to the collection in 1946.

In 1988, a financial crisis in the Diocese of Hereford caused the Dean and Chapter to propose selling the mappa mundi. After much controversy, large donations from the National Heritage Memorial Fund, Paul Getty and members of the public kept the map in Hereford and allowed the construction of a new library designed by Sir William Whitfield to house the map and the chained libraries of the Cathedral and of All Saints' Church. The new Library Building in the south east corner of the cathedral close opened in 1996. An open-access high-resolution digital image of the map with more than 1,000 place and name annotations is included among the thirteen medieval maps of the world edited in the Virtual Mappa project. In 1991 BR Class 31 loco No.31405 was named "Mappa Mundi" at a ceremony at the Hereford Rail Day.

Christopher de Hamel, an expert on medieval manuscripts believes that the map's historical significance cannot be overstated: "... it is without parallel the most important and most celebrated medieval map in any form, the most remarkable illustrated English manuscript of any kind, and certainly the greatest extant thirteenth-century pictorial manuscript." The map was created with the intent of its being appreciated as an intricate work of art rather than as a navigational tool.