Ouvrage Thonnelle, Military fortification in Thonnelle, France.

Ouvrage Thonnelle is a military fortification with four combat blocks positioned along a ridge, each equipped with machine gun stations and observation posts. Underground tunnels connect the blocks and once contained barracks, storage areas, and command centers.

Approved in 1934, the ouvrage was built from 1935 by contractor GTBA for 23.7 million francs, one of four positions in the Tête du Pont de Montmédy salient. On the night of 10–11 June 1940 its garrison of 186 men and 3 officers under Captain Gatellier was ordered to withdraw; the arms and infrastructure were sabotaged and evacuation was complete by 12 June. The German 169th Infantry Division occupied the site on 13 June 1940, and its interior was later stripped by the Germans and by scrap dealers.

The fort's name comes from the nearby village of Thonnelle, and its four blocks were manned in 1940 by 186 men and 3 officers of the 155th Fortress Infantry Regiment under Captain Gatellier. The Casernement de Montmédy provided the garrison's peacetime barracks and support services above ground.

The ouvrage is abandoned, stripped and partly flooded, and entering is reported to be dangerous; view the blocks and cloches from outside rather than going in. The four blocks lie along a hogback ridge, with Block 4 on the steep northern slope, Block 2 at the crest and Block 3 on the gentler southern slope. The site is near the villages of Thonnelle and Verneuil-Petit, a few kilometres from the Belgian border.

Bat colonies now inhabit the underground sections, using the abandoned structure as shelter. This natural reoccupation shows how nature has reclaimed the forsaken facility.