St Albans Cathedral, officially the Cathedral and Abbey Church of St Alban, also known as "the Abbey", is a Church of England cathedral in St Albans, England.

Much of its architecture dates from Norman times. It ceased to be an abbey following its dissolution in the 16th century and became a cathedral in 1877. Although legally a cathedral church, it differs in certain particulars from most other cathedrals in England, being also used as a parish church, of which the dean is rector with the same powers, responsibilities and duties as those of any other parish. At 85 metres long, it has the longest nave of any cathedral in England.

Probably founded in the 8th century, the present building is Norman or Romanesque architecture of the 11th century, with Gothic and 19th-century additions.

According to Bede, whose account of the saint's life is the most elaborate, Alban lived in Verulamium, some time during the 3rd or 4th centuries. At that time Christians began to suffer cruel persecution. The legend proceeds with Alban meeting a Christian priest (known as Amphibalus) fleeing from persecutors, and sheltering him in his house for a number of days. Alban was so impressed with the priest's faith and piety that he soon converted to Christianity. Eventually Roman soldiers came to seize the priest, but Alban put on his cloak and presented himself to the soldiers in place of his guest. Alban was brought before a judge and was sentenced to beheading. As he was led to execution, he came to a fast flowing river, commonly believed to be the River Ver, crossed it and went about 500 paces to a gently sloping hill overlooking a beautiful plain When he reached the summit he began to thirst and prayed that God would give him drink, whereupon water sprang up at his feet. It was at this place that his head was struck off. Immediately after one of the executioners delivered the fatal stroke, his eyes fell out and dropped to the ground alongside Alban's head. Later versions of the tale say that Alban's head rolled downhill and that a well gushed up where it stopped. St Albans Cathedral stands near the supposed site of Alban's martyrdom, and references to the spontaneous well are extant in local place names. The nearby river was called Halywell (Middle English for 'Holy Well') in the medieval era, and the road up to Holmhurst Hill on which the Abbey now stands is now called Holywell Hill but has been called Halliwell Street and other variations at least since the 13th century. The remains of a well structure have been found at the bottom of Holywell Hill. However, this well is thought to date from no earlier than the 19th century.

The date of Alban's execution has never been firmly established. The Anglo-Saxon Chronicle lists the year 283, but Bede places it in 305. Original sources and modern historians such as William Hugh Clifford Frend and Charles Thomas indicate the period of 251–259 (under the persecutors Decius or Valerian) as more likely.

The tomb of St Amphibalus is in the cathedral.

A memoria over the execution point holding the remains of Alban existed at the site from the mid-4th century (possibly earlier). According to Bede, a Christian shrine together with its martyr cult survived from Roman times until the eighth century, although this may be due to British control of this area. Bede also mentions a church and Gildas a shrine. Bishop Germanus of Auxerre visited in 429. This is seen as evidence of the continuation of Christianity in heavily Saxonised south and east Britain. The style of this structure is unknown; the 13th-century chronicler Matthew Paris (see below) said the Saxons destroyed the building in 586.^{[citation needed]}

King Offa of Mercia, is said to have founded a double monastery at St Albans in 793. It followed the Benedictine rule. The Abbey was built on Holmhurst Hill – now Holywell Hill – across the River Ver from the ruins of Verulamium. Again there is no information to the form of the first abbey.