Charles IX of Sweden also Carl, Swedish: Karl IX (4 October 1550 – 30 October 1611) is King of Sweden from 1604 until his death.

He is the youngest son of King Gustav I of Sweden and his second wife, Margaret Leijonhufvud, brother of Eric XIV and John III of Sweden, and uncle of Sigismund III Vasa king of both Sweden and Poland.

By his father's will he got, by way of appanage, the Duchy of Södermanland, which included the provinces of Närke and Värmland; but he did not come into actual possession of them till after the fall of Eric and the succession to the throne of John in 1568.

The Swedish kings Eric XIV (1560–68) and Charles IX (1604–1611) took their numbers according to a fictitious History of Sweden.

This was actually the third Swedish king called Charles.

He comes into the throne by championing the Protestant cause during the increasingly tense times of religious strife between competing sects of Christianity, where forcible conversion is considered a "best course", a period where the Catholics are growing increasingly belligerent— which, in just over a decade, will break out as the Thirty Years' War—as it had already caused the dynastic squabble rooted in religious freedom that deposed his nephew and brought him to rule as king of Sweden.

With his brother's death in November of 1592, during the era which marks the start of the final chapter (dated 1648 by some) of both the Reformation and Counter-reformation, Charles, during the tense political times which prevail, views the inheritance of the throne of Protestant Sweden by his devout Roman Catholic nephew and Habsburg ally, Sigismund of Poland and Sweden, with alarm, and several years of religious controversy and discord follow.

During the period, he along with the Swedish privy council rule in Sigismund's name while he stays in Poland.

After various preliminaries, his nephew is forced to abdicate the throne to Charles IX as regent in 1595 by the Riksens ständer, which eventually kicks off nearly seven decades of sporadic warfare as the two lines of the divided House of Vasa both continue to attempt to remake the union between the Polish and Swedish thrones with opposing counterclaims and dynastic wars.

It is also quite likely that the dynastic outcome between Sweden and Poland's house of Vasa is a factor which exacerbates and radicalizes the later actions of Europe's Catholic princes in the German states such as the Edict of Restitution, and so worsens European politics to the abandonment or prevention of settling events by diplomacy and compromise during the vast bloodletting that is the Thirty Years' war.