Burg Unter-Falkenstein, Medieval castle ruins in Flintsbach am Inn, Germany.

Burg Unter-Falkenstein, also called Neu-Falkenstein, is a hilltop castle ruin at 536.2 m above sea level on the territory of Flintsbach am Inn, at the foot of the Großer and Kleiner Madron. Above it lies the site of Burg Ober-Falkenstein. Preserved are the residential tower, inhabited until 2009, and the foundation walls of the outer bailey.

The castle was built around 1300 following the destruction of its predecessor, Burg Ober-Falkenstein. It then served as a ducal Bavarian court seat for several centuries afterward.

The name Unter-Falkenstein, or Neu-Falkenstein, distinguishes it from the older Ober-Falkenstein castle site above. From 1501 the castle and its court belonged to the Hofer family of Schwaz, who had grown rich from mining, before Duke Wilhelm IV bought it back in 1535. The Preysing lords moved the court seat to Brannenburg, and noble privileges ended with the 1848 revolution.

The ruin stands above Flintsbach am Inn, below the Burgstall of Ober-Falkenstein. Since the June 2024 landslide, when part of the outer bailey wall slid north and 50 residents below were evacuated, the castle is completely closed while the wall is rebuilt. Check locally before setting out.

The residential tower was still lived in until 2009, when the Umwelt-, Kultur- und Sozialstiftung im Landkreis Rosenheim acquired the ruin. Between 2016 and 2019 the site was restored for one million euros. On 3 June 2024, after heavy rain during the southern German floods, part of the outer bailey wall slid northward and 50 residents below had to be evacuated; the castle is currently closed while the wall is rebuilt.