Mareuil-sur-Ay (French pronunciation: [maʁœj syʁ aj], literally Mareuil on Ay) is a former commune in the Marne department in north-eastern France. Since January 2016, Mareuil-sur-Aÿ is part of the administrative commune Aÿ-Champagne. Its population was 1,113 in 2019.

The patron saint is Saint Trésain.

Historical population

Year	Pop.	±%
1962	1,128	—
1968	1,192	+5.7%
1975	1,075	−9.8%
1982	1,159	+7.8%
1990	1,277	+10.2%
1999	1,219	−4.5%
2005	1,139	−6.6%
2010	1,225	+7.6%
2015	1,218	−0.6%
Source: Ldh, EHESS, Cassini until 1999, then Insee from 2006 onwards.

Its vineyards are located in the Vallée de la Marne subregion of Champagne, and are classified as Premier Cru (99%) in the Champagne vineyard classification. Together with Tauxières-Mutry it is the highest rated of the Premier Cru villages, and has therefore just missed out on Grand Cru (100%) status.

Regardless, the vineyards, harvest huts, presses, and cellars in Mareuil-sur-Ay were inscribed on the UNESCO World Heritage List in 2015 as part of the Champagne hillsides, houses and cellars site, because of the region's testimony to the development of champagne.

• Billecart-Salmon
• Communes of the Marne department
• Montagne de Reims Regional Natural Park