The Diocese of Dax or Acqs was a Roman Catholic ecclesiastical territory in Gascony in south-west France. According to tradition it was established in the 5th century. It was suppressed after the French Revolution, by the Concordat of 1801 between First Consul Napoleon Bonaparte and Pope Pius VII. Its territory now belongs to the Diocese of Aire and Diocese of Bayonne.
It is not certain that the patron of the diocese, the martyr St. Vincent, was a bishop. His cult, at least, existed in the time of Charlemagne, as is proved by a note (in a later hand) of the Wolfenbüttel manuscript of the Hieronymian Martyrology. The oldest account of his martyrdom is in a breviary of Dax, dating from the second half of the thirteenth century, but the author knows nothing of the martyr's time period or the reasons for his death.
Excavations near Dax proved the existence of a Merovingian cemetery on the site of a church which, it is claimed, was dedicated to St. Vincent by Bishop Gratianus. Gratianus, present at the Council of Agde (506), is the first historically known bishop. Among the other bishops of the see were St. Revellatus (early sixth century), St. Macarius (c. 1060), Cardinal Pierre Itier (1361), Cardinal Pierre de Foix (1455), founder of the University of Avignon and the Collège de Foix at Toulouse.
The synodal constitutions of the ancient Diocese of Dax, published by Abbé Antoine Degert, are of great historical interest for the study of the ancient constitutions and customs of the thirteenth and fourteenth centuries. Degert in the course of this publication succeeded in rectifying certain errors in the episcopal lists of the Gallia christiana and of Father Eubel in Hierarchia catholica.
During the Great Schism, Dax, which was part of Aquitaine, belonged to the Kings of England (in 1378 Richard II). King Richard chose to support the popes of the Roman Obedience rather than the popes of the Avignon Obedience, who were French and likely to support the King of France in what is now called the Hundred Years' War. All of the cardinals of the Avignon obedience were deprived of their offices and benefices in the Kingdom of Richard II of England, by act of Parliament and decree of the King Dax was required to adhere to the Obedience of Rome.
About 1588 St. Vincent de Paul made his first studies with the Cordeliers of Dax, but good secondary education at Dax dates only from the establishment of the Barnabites in 1640. His learning, however, was sufficient to allow him to study at the University of Toulouse.
On 3 June 1857, the title "Bishop of Dax" was added to the titulature of the Bishop of Aire.