Catherine Jagiellon (Polish: Katarzyna Jagiellonka; Swedish: Katarina Jagellonica, Lithuanian: Kotryna Jogailaitė; 1 November 1526 – 16 September 1583) was a princess of the Polish–Lithuanian Commonwealth and Queen of Sweden from 1569 as the wife of King John III.

Catherine Jagiellon was born in Kraków as the youngest daughter of King Sigismund I the Old of the Polish–Lithuanian Commonwealth and his wife, Bona Sforza of Milan.

Their stay in Vilnius was described as happy, living in a palace and a court strongly influenced by the Italian Renaissance: Catherine and Anna were allowed to compose their own separate households and socialized with the aristocracy.

[2] Next, she was proposed to by Archduke Ferdinand II of Austria, but her brother, who had himself married a member of the Habsburg dynasty by then, ultimately decided against it.

In the 1550s, her brother the king wished for an alliance between Poland and Sweden against the Tsardom of Russia due to growing tension around Livonia.

The Polish-Swedish alliance was a wish her brother entertained for several years, and it had in fact been suggested already in 1526, that time between her half-sister Hedwig and King Gustav I of Sweden.

In August 1560, during the Livonian war, Czar Ivan the Terrible, recently widowed, suggested a marriage between himself and Catherine in order to create peace and settle the differences between Poland and Russia.

On 4 October 1562, Catherine was married in the Lower Castle of Vilnius, Lithuania, to Duke John of Finland, the second son of Gustav I and half-brother of the then-reigning King Eric XIV.

The material dowry she brought with her to Finland, however, greatly impressed her contemporaries: she brought with her an impressive amount of silver items, among them the first forks used in Finland; hundreds of garments in black, yellow, red and purple satin, silk and velvet; as well as an entourage of Poles, Italians and Germans, among them a Polish cook and an Italian vine master.

Tradition claims that when the king made the offer, Catherine pointed to the inscription in her wedding ring, which said: Nemo nisi mors ("Nothing but death [shall separate us]").

[2] She asked for the larger part of her entourage to be sent home, only keeping some Polish ladies-in-waiting and her Court dwarf and personal confidante Dorothea Ostrelska.

Catherine's unsuccessful suitor Tsar Ivan was in negotiations with Eric in hopes of separating her from John and sending her to marry him in Russia.

[1] Her ladies-in-waiting were supervised by Karin Gyllenstierna and her household by chamberlain Pontus De la Gardie, with whom she reportedly had a very good personal relationship (she gave him power of attorney to act as her agent and envoy in Italy regarding her Sforza inheritance).

She received many supplicants from both Catholics and Protestants, asking her for charity as well as to act as mediary to the king, and fulfilled these duties as was expected by a contemporary queen consort.

Her fervent Protestant brother-in-law, the future Charles IX mentioned her in his propaganda chronicle Hertig Karls rimkrönika, in which he slanders the names of her spouse, son and daughter, but with only mild disapproval toward Catherine, acknowledging her personal qualities: "She was a Princess full of virtue and piety, still her faith did come from Rome".

The same year she became queen, her Polish adviser coadjutor Martin Kromer encouraged her to convert John III to Catholicism.

A conflict arose between Catherine and Pope Pius V after it became known that she had received the communion "sub utraque", something which had been banned in the Council of Trent and since then regarded as a sign of heresy.

From 1572, Queen Catherine was in direct contact with Cardinal Stanislaus Hosius, who declared that he would serve as her support and ally in the work of counter-reformation in Sweden and her messenger to the Pope.

This was a sort of mix between Protestantism and Catholicism that reintroduced numerous Catholic customs in the ceremonial life of the Swedish church, one of them being the use of Latin, which aroused a great deal of opposition and resulted in the Liturgical struggle, which was not to end for twenty years.

In 1582, Catherine received the Polish ambassador Alamanni and explained that she was not in a position to convince John to make peace with Poland.

The infusion of Polish - Lithuanian Commonwealth blood into the Swedish royal lineage that began with Catherine would cause considerable strife after her death in the context of the ongoing European wars of religion.

Sigismund and his descendants, as Catholic kings, would continue to lay claim to de facto Protestant Sweden over the following century.

The succession dispute contributed to the outbreak of several destructive wars until a massive Swedish invasion in the 1650s (known as the Deluge) nearly broke up the Polish–Lithuanian Commonwealth.

The religious issues that made Catherine unpopular with her contemporaries were by then long obsolete, and it has instead become traditional to depict her as a compassionate and loyal queen.