Abbaye de Clairmarais, Cistercian abbey ruins in Clairmarais, France.

Abbaye de Clairmarais is a ruined Cistercian abbey in the forest of Rihoult near Saint-Omer. Its surviving fragmentary facade is built of pale gray stone blocks with horizontal bands of red brick, pierced by two large pointed Gothic arches at the base and two empty window openings above; the top edge is broken, with a taller jagged remnant on the upper right. The abbey church and monastery were designed by Geoffroy d'Ainay.

The monastery was established in 1140 and expanded significantly over time to become a major religious center. Economic hardship and religious changes eventually caused the complex to decline and be abandoned.

The name Clairmarais derives from Latin Clarus Mariscus, "clear marsh", after the waterlogged ground where the monks settled. The monks brewed beer here until the abbey closed in 1791; in June 2019 a craft brewery opened in the abbey farm, one of the few surviving buildings, reviving that activity. The abbey church had a carillon of 19 bells.

Visit by guided tour, arranged through the people managing the site, to explore the remnants properly. Wear sturdy footwear since the ground can be muddy due to the wet surroundings.

The monastery raised fish in its marshy waters as an important economic activity, using the wetland environment to its advantage. Traces of the old ponds and channels that served this purpose remain visible in the landscape today.