The castle ruin Karlstein is a castle ruin on a rock in the Bad Reichenhaller district Karlstein in the district of Berchtesgadener Land in Bavaria.
The ruin is registered as an architectural monument in the Bavarian list of monuments, medieval and early modern artefacts are noted as soil monuments.
The ruin of the summit castle is 615 m above the Thumsee on a steep rock and can only be reached via a long metal staircase on the last section of a hiking trail. In front of the castle ruins is today the cath. Pilgrimage Church of St. Pankraz. In place of this little baroque church built between 1687 and 1689, the Pankrazfelsen probably had a bailey in the High Middle Ages.
On the north flank of the rock, directly below today's castle ruins, there were numerous dwellings from the Bronze Age, which are part of the prehistoric settlement areas of Karlstein. On the Haiderburgstein, which rises northeast of the church of St. Pankraz, dwellings from the Bronze and Urnfield Ages were found. The area in which the castle is located today was probably also inhabited at that time. However, since the area was built over in the Middle Ages, there is no archaeological evidence.