Marie Casimire Louise de La Grange d'Arquien (Polish: Maria Kazimiera Ludwika d’Arquien, Lithuanian: Marija Kazimiera; 28 June 1641 – 30 January 1716),[1] known also by the diminutive form "Marysieńka", was a French noblewoman who became the queen consort of Poland and grand duchess consort of Lithuania from 1674 to 1696 by her marriage to King and Grand Duke John III Sobieski of the Polish–Lithuanian Commonwealth.

At the court, she met and fell in love with John Sobieski, who arrived there in 1656, but she was first married off to Jan "Sobiepan" Zamoyski in 1658, with whom she had four children, all of whom died in early childhood.

As the Queen of Poland and Grand Duchess of Lithuania, Marie Casimire quickly became unpopular, as she supported the proposed Polish–French alliance, while simultaneously striving to gain privileges for her family from the French king Louis XIV, whom she greatly admired.

Her influence is exemplified in an incident in 1678, when the Chancellor of Nowogrod, Mikołaj Władysław Przeździecki, came to the king as the leader of an unofficial delegation to present him with evidence that the Pac family were his enemies.

According to Mikolaj Wladyslaw Prze Dziecki, "The Queen knocked on the doors until the King asked for them to be opened and then took the monarch by the hand, leading him to the side, with great force, speaking in French", which the Chancellor viewed with disgust.

[11] When king John Sobieski was absent in Wallachia and Moldova in 1686 and 1691 and at the Battle of Kamenets, queen Marie Casimire are acknowledged to have ruled in effect as regent in his absence.

[12] As such she had the authority to negotiate the military and trade treaty with France in September 1692, which restored the relations between Paris, Copenhagen, Stockholm and Warsaw known as the "Crown of the North Alliance".

[13] She worked with the French ambassador Robert Le Roux d'Esenval and his successor Melchior de Polignac to settle unsettled affairs within the Crown of the North Alliance as well as the Peace treaty with Turkey.

The royal couple became famous for their love letters, most of which were written from 1665 to 1683, when they were parted either due to John III Sobieski's military engagements or her travels to Paris.

The letters give insight not only into the authentic feelings of the loving couple, but also their reflections on contemporary issues and difficulties, as well as down-to-earth matters concerning the royal household and little day-to-day decisions made by the monarch, who often consulted his wife about them.