Charles XI also Carl, Swedish: Karl XI (24 November 1655 – 5 April 1697) was King of Sweden from 1660 until his death, in a period in Swedish history known as the Swedish empire (1611 – 1718).

Charles was the only son of King Charles X of Sweden and Hedwig Eleonora of Holstein-Gottorp. His father died when he was five years old, so Charles was educated by his governors until his coronation at the age of seventeen. Soon after, he was forced out on military expeditions to secure the recently acquired dominions from Danish troops in the Scanian War. Having successfully fought off the Danes, he returned to Stockholm and engaged in correcting the country's neglected political, financial and economic situation, managing to sustain peace during the remaining 20 years of his reign. Changes in finance, commerce, national maritime and land armaments, judicial procedure, church government and education emerged during this period. Charles XI was succeeded by his only son Charles XII, who made use of the well-trained army in battles throughout Europe.

The fact that Charles was crowned as Charles XI does not mean that he was the 11th king of Sweden by the name of Charles. His father's name (as the 10th) was due to his great-grandfather, King Charles IX of Sweden (1604 – 1611), having adopted his own numeral by using a mythological History of Sweden. This descendant was actually the 5th King Charles. The numbering tradition thus begun still continues, with the present king of Sweden being Carl XVI Gustaf.

Charles was born in the Stockholm Palace Tre Kronor in November 1655. His father Charles X of Sweden had left Sweden in July to fight in the war against Poland. After several years of warfare, the king returned in the winter of 1659 and gathered his family and the Riksdag of the Estates in Gothenburg. In mid January 1660 he fell ill and one month later he wrote down his last will and died. Young Charles' education was left at the care of the regents appointed by his father. His mother Queen Hedvig Eleonora was the formal regent until Charles XI attained his majority on 18 December 1672, but she never involved herself much in politics. During his first appearances in parliament, he talked to the government through her: he would whisper the questions he had to her, and she would ask them loud and clear. As an adolescent, Charles devoted himself to sports and exercises, and his favourite pastime bear hunting. He appeared ignorant of the very rudiments of statecraft and almost illiterate. His main difficulties were evident signs of dyslexia, a disability that was poorly understood in those days. According to many contemporary sources, the king was considered poorly educated and therefore not qualified to conduct himself effectively in foreign affairs. Charles was dependent on his mother and advisors to interact with the foreign envoys since he had no foreign language skills apart from a little German and was ignorant of the world outside the Swedish borders.

Italian writer Lorenzo Magalotti visited Stockholm in 1674 and described Charles XI as "virtually afraid of everything, uneasy to talk to foreigners, and not daring to look anyone in the face". Other trait was a deep religious devotion: he was God-fearing, frequently prayed kneeling and attended sermons. Magalotti otherwise described the king's main pursuits as hunting, the upcoming war, and jokes.

The situation in Europe was shaky during this time and Sweden was going through financial problems. The guardians of Charles XI decided to negotiate an alliance with France in 1671. This would ensure that Sweden would not be isolated in case of a war, and that the national finances would improve thanks to French subsidies. France directed its aggression against the Dutch in 1672, and by the spring of 1674, Sweden was forced to take part by directing forces towards Germany, under the lead of Karl Gustav Wrangel. Denmark was an ally of Germany, and it was evident that Sweden was on the verge to yet another war against Denmark. An attempted remedy was made by chancellor Nils Brahe, who traveled to Copenhagen, in the spring of 1675, to try to get the Danish princess Ulrika Eleonora engaged to the Swedish king. In mid June 1675, the engagement was officially proclaimed. However, when news arrived about the Swedish defeat in the Battle of Fehrbellin, Danish king Christian V declared war on Sweden in September.

The Swedish Privy Council continued its internal feuds, and the king was forced to rule without them. The 20-year old king was inexperienced and considered ill-served amidst what has been called the anarchy in the nation and dedicated the autumn in his newly formed camp in Scania to arm the Swedish nation for battle in the Scanian War. The Swedish soldiers in Scania were outnumbered and outequipped by the Danes and, in May 1676, these invaded Scania, taking Landskrona, Helsingborg, and proceeded through Bohuslän towards Halmstad. The King had to grow up quickly: he suddenly found himself alone and under great pressure. The victory at the Battle of Halmstad (17 August 1676), when Charles and his commander-in-chief Simon Grundel - Helmfelt defeated a Danish division, was the king's first gleam of good luck. Charles then continued south through Scania and arrived on the tableland of the flooded Kävlinge River – near Lund – on 11 November. The Danish army commanded by Christian V were positioned on the other side. It was impossible to cross the river and Charles had to wait for weeks until it froze. On 4 December the river froze and Charles launched a surprise attack on the Danish forces to fight the Battle of Lund. This battle was one of the bloodiest engagements of modern times. Of the over 20,000 combatants, about 8,000 perished on the battlefield. All the Swedish commanders showed ability, but the chief glory of the day was attributed to Charles XI and his fighting spirit. The battle proved to be a decisive one for the rule of the Scanian lands and it has been described as the most significant event for Charles' personality; Charles commemorated this date for the rest of his life.

In the following year, 9,000 men lead by Charles routed 12,000 Danes at the Battle of Landskrona. This proved to be the last pitched battle of the war because, in September 1678, Christian V evacuated his army back to Zealand. In 1679, Louis XIV of France dictated the terms of a general pacification, and Charles XI, who is said to have bitterly resented "the insufferable tutelage" of the French king, was forced at last to acquiesce in a peace that managed to leave his empire practically intact. Peace was made with Denmark in the treaties of Fontainebleau (1679) and Lund, and with Brandenburg in the Treaty of Saint-Germain-en-Laye (1679).

Charles devoted the rest of his life avoiding further warfare by gaining larger independence in foreign affairs, while he also promoted the economy stabilization and a reorganization of the military. His remaining 20 years at the throne were the longest peace time of the Swedish Empire (1611 – 1718).

In the first years, he was assisted by the man who had become his trusted prime minister, Johan Göransson Gyllenstierna (1635 – 1680). Some sources say the king was basically dependent on Gyllenstierna, but Gyllenstierna had a big influence upon Charles, at the very least. His sudden death in 1680 opened up the road to the monarch, and many men tried to get close to the king so they could take his place.

Sweden's weak economy had suffered during the war and was now in a deep crisis. Charles assembled the Riksdag of the Estates in October 1680. The assembly has been described as one of the most important held by the Riksdag of the Estates. Here, the king finally pushed through the reduction ordeal, something that had been discussed in the Riksdag since 1650. It meant that any land or object previously owned by the crown and lent or given away — including counties, baronies and lordships — could be recovered. It affected many prominent persons of the nobility class, some of which were ruined. One of them was the former guardian and Lord Chief Justice Magnus De La Gardie, who among many other estates had to return the extravagant 248-room large Läckö Castle. The reduction process involved the examination of every title deed in the kingdom — including the dominions — and it resulted in the complete readjustment of the nation's finances. Another important decision made during the assembly was that of the Swedish Privy Council. Since 1634, it had been mandatory for the king to take the advice from this council. During the Scanian War, the members of the council had internal feuds, and the king more or less ruled without listening to their advice. At the 1680 assembly, he asked the estates whether he was still bound to the council, to which the estates gave him his desired reply: "he was not bound by anyone other than himself", and thereby the absolute monarchy was formally established in Sweden. The Riksdag of the Estates confirmed his power in 1693 by officially proclaiming that the king was the sole ruler of Sweden.

In the 1682 assembly of the Riksdag of the Estates, the king put forth his suggestion for a military reform, whereby each of the lands of Sweden were to have 1,200 soldiers at disposal, at all times, and two farms were to provide accommodations for one soldier. His soldiers were known as Caroleans and were trained to be skilled and to prefer attack to defence. Savaging and looting was strictly forbidden. The soldier huts around the country were the most visible part of the new Swedish allotment system. However, Charles also modernized the military techniques and worked to improve the overall skill and knowledge of the officers by sending them abroad to study. The Swedish Royal Navy was improved with the founding of the Karlskrona naval base in 1680, that became the navy's stronghold since then. It is today a UNESCO World Heritage Site.

Charles felt very important to assimilate the new Swedish dominions of Scania, Blekinge, Halland, in southern Sweden; Bohuslän and Jämtland, in southwest Sweden, and the island of Gotland. Some assimilation policies included: the ban of all books written in Danish or Norwegian, thus breaking the promise made at the Treaty of Roskilde; the use of Swedish language in the conduction of sermons; and all new priests and teachers having to come from Sweden. The king had seen bitter resentment from the Scanian peasants during the Scanian War and was particularly tough on that province. The guerrilla Snapphane movement, in northern Scania, had attacked his soldiers and stolen his money. They also had strong support by the local villages. Charles remained sceptical about the Scanian inhabitants, throughout his life. He did not allow soldiers from Scania in his Scanian regiment: the 1,200 soldiers that were to be stationed in Scania had to be recruited from more northern provinces. He also advocated rough treatment of the inhabitants, and the first Governor - General of Scania, his trusted aide Johan Gyllenstierna (governor - general 1679 – 1680) was notably brutal in his treatment of the locals. The rule of Rutger von Ascheberg (governor - general 1680 – 1693), proved more lenient.

The assimilation was not as strongly implemented on the German dominions of (Swedish Pomerania, the Duchy of Bremen and the Principality of Verden - collectively governed and therefore colloquially called Bremen-Verden - and the Baltic dominions (Estonia and Livonia). In Germany, Charles found himself being opposed by the estates there, and he was also bound by the law of the German emperor and the peace treaty. In the Baltic, the power structure was completely different, with a German descended nobility that used serfs, something that Charles abhorred and wanted to abolish but was unable to. Finally, Kexholm and Ingria were sparsely populated and not of great interest.

Charles was a devoted Lutheran Christian. In February 1686, a church law was put forth on his initiative. The church order declared that the king was ruler of the Church in the same way that he ruled the country and God ruled the world. Attending sermons on Sunday was made obligatory and ordinary people found walking around on the streets, during that time, risked being arrested. Three years later, he declared it obligatory for all commoners to learn to read a catechism — written by archbishop Olov Svebilius and then bishop Haqvin Spegel — so that they would understand the "magnificence of God". Charles then encouraged the production of a hymnal (Psalmbok) to be printed and distributed to the churches (completed 1693), and a new print of the Bible that was completed in 1703 and named after his successor: Charles XII Bible.

On 6 May 1680, Charles married Ulrike Eleonora (1656 – 1693), daughter of King Frederick III of Denmark (1609 – 1670). He had previously been engaged to his cousin, Juliana of Hesse-Eschwege, but the engagement was broken and he married Ulrike after the war, as a part of the peace treaty. Ulrike Eleonora was beautiful and kind, but she always had to stand back to Charles' mother. The Queen Dowager was always mentioned before her in audiences and church blessings, but Ulrika was soft and did not take up the fight. She was completely different from the king: he enjoyed hunting and riding, while she enjoyed reading and art. Her softness was a stark contrast to her husband's roughness. Her Danish background made her situation more difficult; while Charles was away to inspect his troops or pursuing his pastimes, she was often lonely and sad. She gave birth to seven children, of which only three outlived Charles:

• Charles XII (1682–1718), his only son and future king,
• Hedwig Sophia (1681–1708), duchess of Holstein-Gottorp and grandmother of Tsar Peter III, and
• Ulrika Eleonora ("the younger", 1688–1741), who ultimately succeeded her brother on the Swedish throne.

Ulrika (the older) was sickly, and the many child births eventually broke her. When she became seriously ill, in 1693, Charles finally dedicated his time and care to her. Her death in July that year shook him deeply and he never fully recovered. Her infant son Ulric (1684 – 1685) was given Ulriksdal Palace, which was renamed for him (Ulric's Dale).

Charles XI had complained about stomach pains since 1694. In the summer of 1696, he asked his doctors for an opinion on the pain that had gotten continuously worse, but they had no cure for it. He continued his schedule as normal but, in February 1697, the pain became too strong and he had to return to Stockholm, where the doctors discovered he a big hard lump in his stomach. There was little they could do. He died on 5 April 1697, in his forty-first year. An autopsy showed cancer had spread through the entire abdominal cavity.

Charles XI has sometimes been described in Sweden as the greatest of all the Swedish kings, unduly eclipsed by his father and his son. In the first half of the 20th century, the view of him had changed, and he was regarded as dependent, uncertain, and easily influenced by others. In the most recent book, Rystads biography from 2003, the king is again mainly characterized as a strong willed shaper of Sweden through economical reforms and an achievement of financial and military stability and strength.

Charles XI is commemorated on the 500-kronor bill. His portrait is taken from one of Ehrenstrahl's paintings. The reason why the king is on the bill is because the Bank of Sweden was founded in 1668, during Charles' reign.

The fortified town of Carlsburg near Bremen, at the site of modern Bremerhaven, was named after Charles XI.