The Castle Unter-Falkenstein, also called Neu-Falkenstein, is a ruined castle on the territory of the municipality Flintsbach am Inn in the Bavarian district of Rosenheim. The hill fort is located at the foot of the Hohen Madrons and Petersberg.
The castle was founded probably in the 13th Century. After the house of Falkensteiner extincted, the complex was sold in 1556 to George Hundt. The castle burned down in 1784.
Parts of the castle are still existing, among others the mighty, still inhabited castle keep as part of the nucleus and the outer walls.