Leonora Christina, Countess Ulfeldt, born "Countess Leonora Christina Christiansdatter" til Slesvig og Holsten (8 July 1621 – 16 March 1698), was daughter to King Christian IV of Denmark and Kirsten Munk and wife of the Steward of the Realm, the traitor Count Corfitz Ulfeldt. Renowned in Denmark since the 19th century for her posthumously published autobiography Jammers Minde, written secretly during two decades of solitary confinement in a royal dungeon, her intimate version of the major events she witnessed in Europe's history, interwoven with ruminations on her woes as a political prisoner, still commands popular interest and scholarly respect, and has virtually become the stuff of legend as retold and enlivened in Danish literature and art.

Christian IV is believed to have fathered fifteen children by his second wife, Kirsten Munk, at least three of whom were born before the couple married in 1615, and eight of whom lived to adulthood. The Munks were noble courtiers, and Kirsten's formidable mother, née Ellen Marsvin, obtained the King's signed promise to marry the girl before yielding her to the King's passion. The marriage was morganatic and Leonora Christina was not a princess, sharing rather the title of Countess af Schleswig-Holstein bestowed upon her mother in 1629 (distinct from the title borne by the Dukes of Schleswig-Holstein, dynastic kinsmen of the Danish kings who possessed actual domains in the Schleswig and Holstein provinces, some of whom also exercised sovereignty there).

Nonetheless, she grew up with her parents in Copenhagen's royal palace (across the courtyard from the tower where she would eventually be imprisoned) on familiar terms with her three elder half-brothers – including the future King Frederick III – sons of the late Queen Anne Catherine of Brandenburg. She was raised under the supervision of the royal governess Karen Sehested. The King accused his wife of betraying him with another man and divorced her in 1630, having already taken a mistress, Vibeke Kruse, his wife's servant. Although Vibeke Kruse proceeded to bear the King a new brood of children who would become the bitter rivals of Kirsten Munk's children, Leonora Christina seems to have retained her father's favor.

Leonora Christina's marriage was part of Christian IV's strategy to consolidate his dynasty's power. Since 1448 the House of Oldenburg had been Denmark's ruling dynasty, father-to-son. Although hereditary monarchs de facto, until 1660 each successor became king de jure only through election by the Rigsråd. Upon the death of a king, that body would negotiate fresh limitations upon the royal authority, only ratifying the nominee's accession to the throne in return for concessions of rights and privileges. Tradition upheld the King's impartiality and dignity among the nobility by not permitting members of the royal family to marry his subjects, reserving princesses for foreign alliances. But the morganatic status of Leonora Christina and her sisters rendered them useful domestic tools of state, so Christian IV sought to bind the loyalty of powerful or promising nobles by bestowing upon them the hands of these semi-royal daughters, endowed with rich dowries. Six such marriages were arranged. Thus in 1636 the fifteen-year-old Leonora Christina was married to the thirty-year-old Corfitz Ulfeldt, son of the late Chancellor Jacob Ulfeldt, to whom she had been engaged since the age of nine. Though the marriage failed to ensure Ulfeldt's loyalty to the Crown, the young Countess would remain loyal to her husband even beyond his death, accompanying or following him on his every misadventure, and refusing to speak posthumous ill of him even to purchase her freedom.

She and Ulfeldt shared in his renown and initial successes, both at home and abroad. He held the lordships of Egeskov, Hirschholm Urup, Gradlitz and Hermanitz. In 1641 he was made a count (Reichsgraf) by the Holy Roman Emperor Ferdinand III. During most of the 1640s her husband's power and stature grew and she was, in many ways, the first lady of a Danish court that had no queen. Her marriage to Ulfeldt seems to have been a happy one, at least compared to the marriages of her sisters.

At the accession to the throne of her half-brother in 1648, the couple's position was threatened by the resentment of her husband's dominance by Frederick III and, especially, by his queen, Sophie Amalie of Brunswick-Lüneburg, who now became Leonora Christina's relentless enemy. This situation might have been caused both by Leonora's inability to give up her leading position in the court, and by some forms of malice to which she exposed the queen.

At Ulfeldt's disgrace in 1651 (he was rumoured to have been associated with a plot to poison the royal family), she followed him to Amsterdam and Stockholm. They became fugitives, often wandering about to elude capture. She sometimes spent weeks disguised as a man, once fending off arrest from Danish pursuers at gunpoint, and another time the caresses of an infatuated barmaid, the latter proving the more difficult escape. At her insistence, she shared Ulfeldt's exile and expeditions, while he engaged in intrigues with Denmark's enemies for some years, hoping either to return to Copenhagen in power or to humiliate those who held power there. Despite having been made Count of Sölvesborg in Sweden for treasonous services, he was discovered engaged in double treachery and, in 1659, imprisoned. His wife publicly defended him. They escaped separately to Copenhagen where he was promptly arrested, and she shared his harsh imprisonment in the castle Hammershus on the isle of Bornholm 1660–1661, until they ransomed themselves by deeding over most of their properties.

When Ulfeldt was again being sought for treason by the Danes, Leonora Christina went to England to solicit repayment from King Charles II of money her husband had loaned him during his exile. The King repaid his debt by welcoming the Countess (his cousin) to his table, then having her arrested as she boarded a ship to leave England, whereupon he turned her over to Denmark in 1663.