The former city palace of the Dukes of Nassau was built in 1840 as a simple city palace for Duke Wilhelm von Nassau in the late classicist style. It has been the seat of the Hessian state parliament since 1946.

The former residence of the Dukes of Nassau has a magnificent interior, such as wall and ceiling paintings in the style of classical antiquity. After the Second World War, the city palace was chosen as the seat of the state parliament. In 1961, a new plenary building was inaugurated so that the citizens of Wiesbaden and visitors could attend the plenary sessions.

In March 2008, the new plenary hall building was put into operation, offering better working conditions and modern information technology.