Wiesbaden City Palace (German: Stadtschloss Wiesbaden or Wiesbadener Stadtschloss) is a neo-classical building in the center of Wiesbaden, Germany. It was completed in 1841 as the principal city residence of the Dukes of Nassau. The palace has several wings, 145 rooms, and is architecturally integrated with a group of ancillary buildings constructed both before and after it was built. With ornate towers, gables and a slate roof laid in herringbone patterns, the three-story complex lends charm and its name to the central square of Wiesbaden: Palace Square (German: Schloßplatz).

The Palace has had a turbulent history. After withstanding the Revolutions of 1848 and annexation by Prussia in 1866, it has served variously as a second home of the German Emperors, a gathering place for soldiers and workers during the German Revolution of 1918–1919, a museum, and a military headquarters for both the Wehrmacht and Allied occupation forces. Damaged during World War II, the Palace has since been restored and portions modernized. Since 1946 it has been the seat of the State Parliament of Hesse (German: Hessischer Landtag). A new Parliamentary Chamber was added to the complex in 2008.

The site where the City Palace now stands has been the center of political power in Wiesbaden for centuries. It was first occupied by a castle from the Early Middle Ages around which the medieval city developed. By 1236 the Counts of Nassau were already in residence, when there is a record of Holy Roman Emperor Friedrich II celebrating Pentecost in the castle church. During this time the city enjoyed the status of Reichsstadt (Free Imperial City), but only briefly. In subsequent wars Wiesbaden was completely destroyed and rebuilt at least three times. During the Renaissance the city became a notable spa town, and Philipp Ludwig III commissioned construction of a New City Palace, completed in 1599. This renewal was not to last, however. The lands of the House of Nassau were reshuffled in inheritances and as lines died out. The Thirty Years' War again devastated the city, with chronicles recording that by 1648 it had barely 40 residents left. In the 1700s, Schloss Biebrich (the "Versailles on the Rhine" on the outskirts of Wiesbaden) became a principal residence of the line of Nassau-Idstein and the City Palace fell into disuse. Little is known about the extent of the old original Castle, but remains of a tower dating back as early as the Frankish period (481–800 CE) were discovered in 1952 during reconstruction of adjacent buildings damaged in the Second World War.

In 1806, the counties of Nassau-Usingen and Nassau-Weilburg merged to form the Duchy of Nassau at the insistence of Napoleon. Frederick Augustus, Duke of Nassau, became the ducal head of state. After the defeat of Napoleon, the Duchy of Nassau joined the German Confederation at the Congress of Vienna in 1815. The capital moved from Weilburg to Wiesbaden and the city became the ducal residence. When Frederick died childless in 1816, the dukedom was transferred to the line of Nassau-Weilburg. His cousin Wilhelm of Nassau-Weilburg became Wilhelm, Duke of Nassau and moved from Weilburg into Schloss Biebrich on the outskirts of Wiesbaden.

By 1826, extensive building began to give the new capital city a magnificent appearance and in 1830 plans were laid to move the ducal residence into the city. Initially, a site on the Luisenplatz was considered, which was then still on the outskirts. However, the Duke felt it important to live amidst his subjects, so a central location on the Market Square was decided upon. In 1835, Wilhelm I engaged his Senior Architect Georg Moller to design the new City Palace. Several buildings were acquired on the north side of the square and in 1837 the foundation stone was laid. Duke Wilhelm would not live to its completion; He died of a stroke on 20 August 1839 while "taking the cure" at Bad Kissingen. His son Adolph I, Duke of Nassau became the Palace's first and only ducal resident in 1841. He lived there during the winter and in summers resided at Schloss Biebrich.

During the Revolutions of 1848 a national uprising began in many German States. On 4 March 1848 it spread to the Duchy, when a third of the male population of some 30,000 angry citizens gathered in front of the City Palace to demand confirmation of the Nine Claims of Nassau, an early Bill of Rights. With the crowd threatening to storm the Palace, Adolph rushed back to Wiesbaden from Berlin, where he had been consulting on the crisis. He rescued his monarchy at the last minute by walking, unguarded and in uniform, through the crowds from the railroad station to the City Palace. He announced his assent to their demands from the balcony and the menacing riot turned into a celebration. In April 1848, the Duke appointed the liberal politician August Hergenhahn as his Prime Minister and several advanced laws were enacted. While Adolph did not fulfill all his promises, and some were not adopted until 1861, by enacting important reforms he enjoyed a longer reign and the respect of the population.

In 1866, Adolph fatefully supported the Austrian side in the Second Austro-Prussian War, which lost. The Kingdom of Prussia annexed the Duchy as a war prize, sent the Duke into exile and took possession of the Wiesbaden City Palace. It was then used as a residence by Emperor Wilhelm I and the adjacent Kaiser-Wilhelm Sanatorium, built in 1871 as a military hospital, was named in his honor. Wilhem's grandson, Kaiser Wilhelm II, then made the palace a regular summer home. He resided there annually with his staff and members of the Hohenzollern family. Wiesbaden experienced tremendous growth and became an international resort, with the population increasing from 35,500 in 1871 to 100,000 by 1905. Near the Palace numerous new important buildings were constructed, including a New Town Hall (1897), the Hessian State Theater (1894), the Kurhaus (1907) and the Central Railway Station (1906).

After Germany's defeat in World War I and the monarchy collapsed, the palace was used for a variety of purposes. It initially became home to the local Workers' and Soldiers' Council during the German Revolution of 1918-1919. Later it was used as the supreme headquarters of the French Occupation Armies during the Allied Occupation of the Rhineland. In 1925, the palace became the headquarters of British Army of the Rhine. After the withdrawal of occupying forces in 1930, the building came into the possession of the Prussian State Administration of Palaces (German: Preußischen Staatlichen Schlösserverwaltung) and was converted into a museum.

During World War II, the building complex became the Wehrmacht's General Headquarters for Wehrkreis (Military District) XII under the command of General der Infanterie Walther Schroth. This region included the Eifel, part of Hesse, the Palatinate, and the Saarland. After Germany's victory in the Battle of France in 1940, Wehrkreis XII was expanded to include the Grand Duchy of Luxembourg and the Lorraine Region, including Nancy. In the immediate aftermath of World War II, the US Army used the partially ruined building as its headquarters.

In 1946, Wiesbaden became the capital of the newly created State of Hesse and the Palace became the seat of the Hessian Landtag. The Landtag met for the first time in the largest room of the House, the Music Hall, on 1 December 1946. In 1959 the former Royal Riding Hall in the courtyard was demolished and a new Plenary Chamber built in its place. The Palace's historic rooms were preserved and used for receptions. Ancillary buildings around the Palace were redeveloped for use by the Landtag administration. In 1975 the editorial staff of Hessian Radio (German: Hessischer Rundfunk) moved into the complex to better cover State politics and also opened a TV studio there.

In the late 1990s plans began to construct a new building to replace the outdated Plenary Chamber. The Bundestag wanted to build a transparent glass Hall on the Schlossplatz between the Palace and the Marktkirche in the architectural style of the Federal buildings in Bonn. That project was halted due to cost as well as protests from the population about the building's appropriateness. The Federal Design Office then advertised an architectural competition and Waechter + Waechter of Darmstadt won. Their proposal for a low-profile building inside the courtyard behind the Palace's facade was designed preserve the historic character of the Schloßplatz. This was approved and the old Plenary Chamber was demolished in 2004. The City Palace became a construction site for the next four years and on 4 April 2008 parliament moved into its new home.

The palace complex serves as a place of work for the 118 elected MPs of the Hessian State Parliament. The historic rooms of the ducal palace itself are used for official receptions of President of the Parliament (German: Landtagspräsident), as well as for other celebrations. The former Music Hall is also used for public events and concerts. In the adjacent buildings – Gentleman's House, Wilhelmsbau and the former Administrative Court – the MP's offices and State Parliament administration are housed. There are also meeting rooms for committees and facilities for press conferences. The Parliament restaurant is housed in the basement. The Hessian State Government chaired by the Minister-President also has rooms which can be used during the plenary sessions. Additionally there are facilities for accommodating visitors who participate in the public meetings of the Landtag. The plenary hall has a large foyer, a public gallery, and an information center on the history of the Parliament. Today the Wilhelmbau houses the Wiesbaden studios of Hessischer Rundfunk and a well-known room used for national press conferences.