St. John' Church, Mainz, Carolingian church in Mainz-Altstadt, Germany

St. Johannis is a three-nave Carolingian basilica in Mainz's old town, its wide central nave rising above narrow aisles. The east choir is as wide as the nave, its side walls opened by two arched windows with round oeil-de-boeuf windows above. The white-plastered exterior has a tall slate roof with dormers and two towers, one with a pyramidal top, the other with louvered openings.

Archbishop Hatto I rebuilt the cathedral church here around 900 and consecrated it in 910; it served as Mainz's episcopal see until Willigis began the new cathedral east of it after 975, consecrated in 1036. King Henry II was crowned here in 1002, and Archbishop Erkanbald was buried here in 1021. A canon's collegiate foundation was then installed, and the patronage of John the Baptist is documented from 1128.

The name Aldeduom (Old Cathedral) survives from the centuries after 1036, when the new Mainz Cathedral took over the bishop's seat and this church became a canons' collegiate foundation. Its patron is John the Baptist, documented in 1128, though the dedication was probably set in 1036. The Mainz cathedral chapter still held a service here on St Martin's Day in the 18th century.

The church stands directly beneath Mainz Cathedral in the Altstadt, a short walk from the Markt. The floor inside lies about 2.60 metres above the level of Hatto's time, the result of debris layers built up over centuries. Visitors enter through the red arched doorway on the left of the street facade; the interior is reached past the open entrance with the small stone statue.

The floor level has risen about 2.60 metres since Hatto's time, so the present floor is probably the seventh in the building's history. In places, tuff masonry from the 6th or 7th century survives nine metres up the walls, hidden under plaster until researchers from Heidelberg removed it.