Baulne-en-Brie is a former commune in the department of Aisne in the Nord-Pas-de-Calais-Picardie region of northern France. On 1 January 2016, it was merged into the new commune Vallées-en-Champagne.
Baulne-en-Brie is located some 15 km east by south-east of Château-Thierry and 20 km west by south-west of Épernay. It can be accessed by the D4 road from Condé-en-Brie in the west through the heart of the commune and the village and continuing east to Le Breuil. There is also a country road from La Chapelle-Monthodon in the north. The south-eastern border of the commune is part of the border between the departments of Aisne and Marne. There are three hamlets in the commune other than the village: Montchevret, Grande Fontaine, and Romandie. There are extensive forests in the north, centre, south-east and south of the commune with the rest of the area being farmland.
The Verdonnelle stream flows north-west through the southern part of the commune passing near the hamlet of Romandie and continuing north-west out of the commune. The Surmelin stream flows from the south-east passing through the village and continuing north-west to join the Marne near Mézy-Moulins.