Katharina von Bora (German: [kataˈʁiːnaː fɔn ˈboːʁaː]; 29 January 1499? – 20 December 1552), after her wedding Katharina Luther, also referred to as "die Lutherin" ('the Lutheress'), was the wife of the German reformer Martin Luther and a seminal figure of the Protestant Reformation. Although little is known about her, she is often considered to have been important to the Reformation, her marriage setting a precedent for Protestant family life and clerical marriage.

Katharina von Bora was the daughter to a family of Saxon lesser nobility. According to common belief, she was born on 29 January 1499 in Lippendorf, but there is no evidence of this in contemporary documents. Due to there being multiple branches in her family and the uncertainty of her birth name, there are diverging theories about her place of birth. One of them proposes that she was born in Hirschfeld and that her parents were Hans von Bora zu Hirschfeld and his wife, born Anna von Haugwitz. It is also possible that Katharina was the daughter of Jan von Bora auf Lippendorf and his wife Margarete, both of whom were only mentioned in 1505.

Her father sent five-year-old von Bora to a Benedictine convent in Brehna in 1504 to be educated, according to a letter Laurentius Zoch sent to Martin Luther in 1531. At the age of nine, she was moved to Nimbschen Abbey, Cistercian community named Marienthron ('Mary's Throne') near Grimma, where her maternal aunt was a nun. Von Bora's presence is in the financial accounts of 1509/10.

After years of being a nun, von Bora became interested in the growing reform movement and grew dissatisfied with cloistered life. Conspiring with several other sisters, she contacted Luther and begged for his assistance. On 4 April 1523, Holy Saturday, Luther sent Leonhard Köppe, a merchant and councillor of Torgau who regularly delivered herring to the convent. The nuns escaped by hiding in his covered wagon among the fish barrels, and fled to Wittenberg.

Luther asked the families of the nuns to admit them into their houses, but they declined, possibly because this would have made them accomplices to a crime under canon law.

Within two years, Luther was able to arrange marriages or find employment for all of the escaped nuns except von Bora. She was first housed with the family of Philipp Reichenbach, the municipal clerk of Wittenberg, then with Lucas Cranach the Elder and his wife, Barbara. Von Bora had a number of suitors, including Hieronymus Baumgartner from Nuremberg, and a pastor, Kaspar Glatz from Orlamünde, but none of the proposals resulted in marriage. She told Luther's friend and fellow reformer, Nicolaus von Amsdorf, that she would be willing to marry only Luther or von Amsdorf.

Martin Luther, as well as many of his friends, was at first unsure of whether he should marry. Philip Melanchthon thought that this would hurt the Reformation by causing scandal. Luther eventually decided that his marriage would 'please his father, rile the pope, cause the angels to laugh, and the devils to weep'. 26-year-old Von Bora and 41-year-old Luther married on 13 June 1525, before witnesses including Justus Jonas, Johannes Bugenhagen, and Barbara and Lucas Cranach. A small wedding breakfast was held the next morning, and a more formal, public ceremony on 27 June, presided over by Bugenhagen.

The couple took up residence in the former dormitory and educational institution of Augustinian friars studying in Wittenberg (known as the 'Black Monastery'), a wedding gift from John, Elector of Saxony, brother of Luther's protector Frederick III, Elector of Saxony. Katharina immediately took on the task of managing the monastery's vast holdings. She bred and sold cattle and ran a brewery to provide for their family, the numerous students who boarded with them, and her husband's visitors. In times of epidemics, she operated a hospital with nurses, working alongside them. Luther called her the 'boss of Zulsdorf', after the farm they owned, and the 'morning star of Wittenberg' for her habit of rising at 4 a.m.