Karl, Duke of Schleswig-Holstein-Sonderburg-Glücksburg (30 September 1813 – 24 October 1878) was Duke of Schleswig-Holstein-Sonderburg-Glücksburg from 1831 to 1878. Karl was the eldest son of Friedrich Wilhelm, Duke of Schleswig-Holstein-Sonderburg-Glücksburg, and Princess Louise Caroline of Hesse-Kassel and an elder brother of Christian IX of Denmark.

Upon his father's death in 1831, Karl inherited Glücksburg Castle and became Duke of Schleswig-Holstein-Sonderburg-Glücksburg at the age of seventeen. In 1838, he became the son-in-law of King Frederick VI of Denmark when he married the his cousin, Princess Vilhelmine Marie of Denmark. A supporter of the Schleswig–Holstein movement, Duke Karl actively sided against Denmark during the First Schleswig War from 1848 to 1851, which caused the duke's relations with the Danish Royal family to be severed. The ducal couple lived abroad until an uneasy reconciliation was established in 1852. In 1854, he had to cede the family seat Glücksburg Castle to King Frederick VII of Denmark. During the Second Schleswig War in 1864, the ducal couple left the country again, only to return again the following year after the Prussian victory. In 1871, after long negotiations with the Prussian Government, the Duke again took possession of Glücksburg Castle. Upon the death of the childless duke, the ducal title was inherited by his younger brother, Friedrich.

On 19 May 1838, the 24 year old Duke Karl married his first cousin, the five year older Princess Vilhelmine Marie of Denmark, daughter of Frederick VI of Denmark and his wife Marie Sophie of Hesse-Kassel, at Amalienborg Palace in Copenhagen.^[5] Vilhelmine Marie was the former wife of Prince Frederick of Denmark (later Frederick VII of Denmark), whom she divorced in 1837.^[6] After the wedding, the newly married couple moved into the castle in Kiel which the king gave them as their residence.^[1]

During the Schleswig–Holstein question, Duke Charles was a supporter of the Schleswig–Holstein movement. In 1846, as head of the House of Glücksburg, he protested against King Christian VIII's open letter on the succession in the duchies of Schleswig and Holstein, and out of dissatisfaction resigned from the Army in August of the same year.^[5] Nevertheless, eight days after his accession to the throne, King Frederick VII gave him the rank of major general.^[6] Duke Karl actively sided against Denmark during the First Schleswig War from 1848 to 1851, which caused the ducal couple's relations with the Danish Royal family to be severed. During the war, he initially took command of one of the provisional Schleswig-Holstein government's Infantry Brigades, but already in the autumn of 1848, he and his wife moved to Dresden in the Kingdom of Saxony.^[5] There they resided until 1852, when they returned to Denmark after am uneasy reconciliation was established.^[6] As a result of his rebellion against the Danish government, Duke Carl lost his Danish orders and decorations and only had them restored in 1856.^[5] Also, in 1854, he had to cede the family seat Glücksburg Castle to King Frederick VII of Denmark who used the castle as his summer residence and also died there in 1863.^[7]

In the following years, they lived at Kiel Castle in the winter, and in the summer at Louisenlund near the town of Schleswig.^[6] During the Second Schleswig War of 1864, they left the country again, only to return again the following year after the Prussian victory in the war.^[6] In the subsequent years, they lived at Louisenlund, but no more in Kiel.^[6] In 1871, however, the Duke and Duchess again took possession of the family seat Glücksburg Castle which after long negotiations in 1870 was returned to the Duke by the Prussian Government, and which they then used as their principal residence.^[7]

Duke Karl died at the age of 65 on 24 October 1878 at Glücksburg Castle.^[1][5][6] His marriage with Vilhelmine Marie was without issue,^[6] and many believe that she was barren as there are no records of her having any miscarriages or stillbirths. Upon his death, the ducal title was therefore inherited by Duke Karl's younger brother, Friedrich. Duke Karl was interred at Glucksburg Castle.^[5] Duchess Vilhelmine Marie survived her husband by 12 years and died on 30 May 1891 at Glücksburg Castle.