Aleth Cathedral, Ruined pre-Romanesque cathedral in Saint-Servan, Saint-Malo, France.

Aleth Cathedral is the ruin of a Gothic church standing on a peninsula at the mouth of the Rance estuary and shaping this coastal landscape. Its stone walls still stand in parts, showing the characteristic features of Gothic architecture with its arches and structural forms.

Built around 920, the cathedral reproduced the plan of old Saint Peter's in Rome, with two opposed apses, a 35.75 m nave on eight bays and a width of 15.50 m. The see was moved to Saint-Malo in 1146 by Bishop Jean de Châtillon. In 1255 a tax conflict with Saint-Malo ended in war; Aleth lost and its walls, fortress and church were dismantled. The east apse became a chapel to Saint Peter, restored in 1868.

The name Aleth survives in the quartier and the corniche; the eastern apse was fitted out as a chapel dedicated to Saint Peter and restored in 1868. Excavations by the regional archaeology centre of Alet between 1972 and 1978 uncovered the plans of the successive buildings, and the site today shows the consolidated foundations of the pre-Romanesque cathedral.

The site is accessible via a coastal pathway called Corniche d'Aleth, which leads visitors along the shore and connects to other historical locations nearby. The walk offers good views and allows you to see the ruin from different angles.

The location on the Aleth peninsula was so strategically important that it later shaped the development of Saint-Malo as a fortified port city. This position made the site a crucial control point for the entire region.