The first mention of the fortress situated high up on a protruding rock, known itself until the 16th century as ,Stein' (rock), can be documented for the year 1191 when the ministerial official Gerhard von Nöbdenitz and his mother Mechthilde von Stein were mentioned in a Naumburg document. Fortress and village evidently originated from a clearing of the surrounding woods initiated by the population of the neighbouring Nöbdenitz which had already been settled when the area was Slavic, in the second half of the 12th century.  

Since the beginning of the 14th century, the by that time very strong dynasty of the Reußen which had expanded its holdings all the way to the Pleißenland, is documented as feudal lord over Posterstein and until the 17th century, the  Wettiner dynasty and the Reußen fought over Posterstein. Only with its diminishing importance in the 18th century did this end.  

In 1305, at the time when the dynasty of the Reußen split into the older Reuß line (in Plauen) and the younger Reuß line (in Greiz), Posterstein remained under the domain of the younger line.  

In the 15th century, the property ended up in the Puster family, who according to the language usage of the times were called the „Postern from Stein", indicating how the name of the castle, used since the 16th century, has come about.  The Pusters served both the Wettiner and the Reußen as knights.

In 1528, the Pflugk family took over the fortress, a family who through administrative politics and marriage politics managed to considerably secure and expand the property rights.  In the 17th century, the fortress which had suffered much from war damages and wear was remodelled and the adjacent chapel was adorned with baroque carvings.  

After a few ownership changes (the brothers von Werder, the imperial earls Flemming), in 1833 the middle class (untitled) Herrmann family acquired the knight's castle Posterstein.  Soon after commenced the negotiations between farmers and the lords of the manor, which in 1850 led to the ceasing of compulsory service through payment of a sum of money.  With the suspension of the jurisdiction feudal society was abolished in the area.  

By the end of the 19th century Posterstein managed to reach fame as an exemplary agricultural estate in the Thuringia region.  In 1913, the excellent education and training facilities attracted also Rudolf Ditzen who later became a famous writer under the name Hans Fallada.  

In the course of the land reform in 1946 the knight's castle Posterstein and its 474 acres of land were expropriated. Since that time the castle is under public administration and the founding of the museum in 1952 made possible the preservation of the property which is on the historic register. Between 1984 and 1991 the castle was restored with funds provided from the offices for historic preservation. In 1991 the museum reopened with a new concept and newly designed exhibits.