Speyer Cathedral, officially the Imperial Cathedral Basilica of the Assumption and St Stephen, in Latin: Domus sanctae Mariae Spirae (German: Dom zu Unserer lieben Frau in Speyer) in Speyer, Germany, is the seat of the Roman Catholic Bishop of Speyer and is suffragan to the Roman Catholic Archdiocese of Bamberg. The cathedral, which is dedicated to St. Mary, patron saint of Speyer ("Patrona Spirensis") and St. Stephen is generally known as the Kaiserdom zu Speyer (Imperial Cathedral of Speyer). Pope Pius XI raised Speyer Cathedral to the rank of a minor basilica of the Roman Catholic Church in 1925.

Begun in 1030 under Conrad II, with the east end and high vault of 1090–1103, the imposing triple-aisled vaulted basilica of red sandstone is the "culmination of a design which was extremely influential in the subsequent development of Romanesque architecture during the 11th and 12th centuries". As the burial site for Salian, Staufer and Habsburg emperors and kings the cathedral is regarded as a symbol of imperial power. With the Abbey of Cluny in ruins, it is the largest remaining Romanesque church and building. It is considered to be "a turning point in European architecture", one of the most important architectural monuments of its time and one of the finest Romanesque monuments.

In 1981, the cathedral was added to the UNESCO World Heritage List of culturally important sites as "a major monument of Romanesque art in the German Empire".

In 1025, Conrad II ordered the construction of the Christian Western world's largest church in Speyer which was also supposed to be his last resting place. Construction began in 1030 on the site of a former basilica which stood on an elevated plateau right by the Rhine but safe from high water. Along with Santiago de Compostela (begun 1075), Cluny Abbey (Cluny III, begun 1085), and Durham Cathedral (begun 1093), it was the most ambitious project of the time. The red sandstone for the building came from the mountains of the Palatine Forest and is thought to have been shipped down the channelled Speyerbach, a stream running from the mountains into the Rhine at Speyer. Neither Conrad II, nor his son Henry III, were to see the cathedral completed. Conrad II died in 1039 and was buried in the cathedral while it was still under construction; Henry III was laid next to him in 1056. The graves were placed in the central aisle in front of the altar.

Nearly completed, the cathedral was consecrated in 1061. This phase of construction, called Speyer I, consists of a Westwerk, a nave with two aisles and an adjoining transept. The choir was flanked by two towers. The original apse was round inside but rectangular on the outside. The nave was covered with a flat wooden ceiling but the aisles were vaulted, making the cathedral the second largest vaulted building north of the Alps (after Aachen Cathedral). It is considered to be the most stunning outcome of early Salian architecture and the "culmination of a design which was extremely influential in the subsequent development of Romanesque architecture during the 11th and 12th centuries".

Around 1090, Conrad's grandson, Emperor Henry IV, conducted an ambitious reconstruction in order to enlarge the cathedral. He had the eastern sections demolished and the foundations reinforced to a depth of up to eight metres. Only the lower floors and the crypt of Speyer I remained intact. The nave was elevated by five metres and the flat wooden ceiling was replaced with a groin vault of square bays, one of the outstanding achievements of Romanesque architecture. Each vault extends over two bays of elevation. Every second pier was enlarged by adding a broad pilaster or dosseret, which formed a system of interior buttressing. Engaged shafts had appeared around 1030 in buildings along the Loire (Saint-Benoît-sur-Loire, Auxerre, Loches) from where the technique spread to Normandy and the Rhineland.

The only other contemporary example of such a bay system is in the Church of Sant Vicenç in Cardona, Spain. The "double-bay system" of Speyer functioning as a support for the stone vaults was copied in many monuments along the Rhine. The addition of groin vaults made the incorporation of clerestory windows possible without weakening the structure. "The result is an interior of monumental power, albeit stark and prismatic when compared with contemporary French buildings, but one which conveys an impression of Roman gravitas, an impression singularly appropriate for a ruler with the political pretensions of Henry IV."

In the course of these modifications the cathedral was equipped with an external dwarf gallery, an arcaded gallery recessed into the thickness of the walls, and which is a natural development of the blind arcade. Such blind arcades were used extensively as decorations, lining internal and external walls of many Romanesque churches. At the east end of Speyer Cathedral the dwarf gallery and the blind arcades were composed into "one of the most memorable pieces of Romanesque design". The dwarf gallery encircles the top of the apse, underlining its rounded form, and runs all around the structure below the roofline. This feature soon became a fundamental element in Romanesque churches; it was adopted at Worms Cathedral and Mainz Cathedral, and on the façades of many churches in Italy (see detail in gallery below). "The cathedral re-emerged in a more sculptural style typical of the prime of the Romanesque period." "The transept, the square of the choir, the apse, the central tower and the flanking towers were combined in a manner and size surpassing anything done before. All surfaces and edges rise without stages. The major elements within the combination remain independent.... Speyer became a model for many other church buildings but was unsurpassed in its magnificence."