Charles XV also Carl (Carl Ludvig Eugen); Swedish: Karl XV and Norwegian: Karl IV was King of Sweden (Charles XV) and Norway, there often referred to accurately as Charles IV, from 1859 until his death. Though known as King Charles XV in Sweden (and also on contemporary Norwegian coins), he was actually the ninth Swedish king by that name, as his predecessor Charles IX (reigned 1604–1611) had adopted a numeral according to a fictitious history of Sweden.

He was born in Stockholm Palace, Stockholm in 1826 and dubbed Duke of Scania at birth. Born the eldest son of Crown Prince Oscar of Sweden and his wife Crown Princess Josephine, he would be second in line to the throne of his grandfather, the ruling King Charles XIV John of Sweden. During his childhood he was placed in the care of the royal governess countess Christina Ulrika Taube. When he was just 15, he was given his first officer's commission in 1841 by his grandfather the king.

The aging King Charles XIV John would suffer a stroke on his 81st birthday in 1844, dying little more than a month later. His successor would be his son, Charles's father Oscar, who ascended the throne as King Oscar I of Sweden. Upon his father's accession to the throne in 1844, the youth Charles was made a chancellor of the universities of Uppsala and Lund, and in 1853 chancellor of Royal Swedish Academy of Arts. On 11 February 1846 he was made an honorary member of the Royal Swedish Academy of Sciences.

The Crown Prince was Viceroy of Norway briefly in 1856 and 1857. He became Regent on 25 September 1857, and king on the death of his father on 8 July 1859. As grandson of Augusta of Bavaria, he was a descendant of Gustav I of Sweden and Charles IX of Sweden, whose blood returned to the throne after being lost in 1818 when Charles XIII of Sweden died.

On 19 June 1850 he married in Stockholm Louise of the Netherlands, niece of William II of the Netherlands through her father and niece of William I of Prussia, German Emperor, through her mother. The couple were personally quite dissimilar; Louise was a cultured and refined woman, however, she was considered to be quite plain and Charles was disappointed with her appearance. Louise was in love with her husband, whereas he preferred other women, saddening her deeply. His well-known mistresses included the actress Laura Bergnéhr, the countess Josephine Sparre, Wilhelmine Schröder and the actresses Hanna Styrell and Elise Hwasser, and the Crown Prince neglected his shy wife. On the other hand, his relationship to his only daughter, Louise, was warm and close.

As Crown Prince, Charles's brusque manner led many to regard his future accession with some apprehension, yet he proved to be one of the most popular of Scandinavian kings and a constitutional ruler in the best sense of the word. His reign was remarkable for its manifold and far-reaching reforms. Sweden's existing municipal law (1862), ecclesiastical law (1863) and criminal law (1864) were enacted appropriately enough under the direction of a king whose motto was: Land skall med lag byggas – "With law shall the land be built". Charles also helped Louis De Geer to carry through his reform of the Parliament of Sweden in 1866. He also declared the freedom of women by passing the law of legal majority for unmarried women in 1858 – his sister Princess Eugenie became the first woman who was declared mature.

Charles, like his father Oscar I, was an advocate of Scandinavianism and the political solidarity of the three northern kingdoms, and his friendship with Frederick VII of Denmark, it is said, led him to give half promises of help to Denmark on the eve of the war of 1864, which, in the circumstances, were perhaps misleading and unjustifiable. In view, however, of the unpreparedness of the Swedish army and the difficulties of the situation, Charles was forced to observe a strict neutrality. He died in Malmö on 18 September 1872.

Charles XV attained some eminence as a painter and as a poet. He was followed on the thrones of both Norway and Sweden by his brother Oscar II.

In 1872, Charles XV had controversial plans to enter a non-morganatic marriage with the Polish countess Marya Krasińska through the assistance of Ohan Demirgian, plans that aroused opposition both in the royal house and government and which were interrupted only by his death.