Catherine of Bosnia (Serbo-Croatian: Katarina Kosača/Катарина Косача; 1424/1425 – 25 October 1478) was Queen of Bosnia as the wife of King Thomas, the penultimate Bosnian sovereign. She was born into the powerful House of Kosača, staunch supporters of the Bosnian Church. Her marriage in 1446 was arranged to bring peace between the King and her father, Stjepan Vukčić. The queenship of Catherine, who at that point converted to Roman Catholicism, was marked by an energetic construction of churches throughout the country.

Following her husband's death in 1461, Catherine's role receded to that of queen dowager at the court of her stepson, King Stephen Tomašević. Two years later, forces of the Ottoman Empire led by Mehmed the Conqueror invaded Bosnia and put an end to the independent kingdom. Catherine's stepson was executed, while Sigismund and Catherine, her son and daughter by Thomas, were captured and taken to Constantinople, where they converted to Islam. Queen Catherine escaped, taking refuge in Dubrovnik and eventually settling in Rome, where she received a pension from the papacy. From Rome she strove to be reunited with her children. Her efforts to negotiate and offer a ransom proved futile. She died a Franciscan tertiary in Rome, having named the papacy guardians of Bosnia and her children heirs to the throne, should they ever return to Christianity.

Queen Catherine remains one of the most important figures in the folk tradition and history of Bosnia and Herzegovina. She has long been venerated by the Catholics of Bosnia and Herzegovina, and is increasingly seen as an important transethnic state symbol.

Catherine was the daughter of Stjepan Vukčić Kosača, Grand Duke of Bosnia and Duke of Hum and Primorje, Knyaz of Drina, usually considered by historiography to be the most powerful figurehead of his contemporaries amongst the Bosnian nobility. His domain later came to be known as Herzegovina, after the German title of herzog, which he later adopted in relation to the Humska Zemlja (transl. Land of Hum). Catherine's mother was Jelena Balšić, daughter of Zeta's lord Balša III and Mara Thopia of Albania. Jelena was the first of Stjepan's three wives. Catherine was the couple's first child but the precise date of her birth is not known; both 1424 and late August 1425 have been suggested by her biographers. It can be surmised that the place of her birth was either the Sokol Fortress, one of the residential fortresses of the House of Kosača, or the feudal town of Blagaj, Stjepan's favourite residence.

Little is known about Catherine's premarital life. The earliest source that mentions her is the will of her maternal great-grandmother Jelena Lazarević, dated 25 November 1442, in which she left her her gold earrings and a snake-shaped bracelet. Stjepan was a member and significant supporter of the Bosnian Church, while her mother was an Eastern Orthodox Christian; Catherine was raised in her father's confession.

Catherine comes into focus following the accession of Stephen Thomas to the Bosnian throne in 1443. Thomas, of illegitimate birth but designated as heir by Tvrtko II, belonged to the Bosnian Church and was married according to its customs. His wife, Vojača, was the mother of his son, Stephen Tomašević. Two years after his accession, King Thomas abandoned the Bosnian "heresy" and converted to Roman Catholicism. The Catholic Church did not recognize his union with Vojača as a valid marriage, and Pope Eugene IV gave him permission to repudiate her in early 1445. The civil war between the King and Catherine's father, which had been raging since the former's enthronement, ended soon afterwards. Peace was to be sealed with the marriage of King Thomas and Catherine, a great honour to her father. The project may have been envisioned as early as the beginning of 1445, when Thomas requested the annulment of his union with Vojača.

The marriage ceremony was conducted according to the Catholic rite between 19 May, when Catherine arrived in Milodraž near Fojnica accompanied by her father, and 22 May 1446. The wedding was attended by a delegation from the Republic of Ragusa (Dubrovnik), but it failed to end all internal strife, since leading noblemen such as Ivaniš Pavlović (Catherine's first cousin, lord of eastern Bosnia) and Petar Vojsalić (lord of Donji Kraji) snubbed it. The coronation, planned to take place in Mile near Visoko immediately after the wedding, was apparently postponed. The new queen consort converted to Catholicism ("laid aside Patarin errors"), likely prior to her marriage, and was allowed by Pope Eugene to choose for herself two chaplains from among Bosnian Franciscans. The Queen's father appears to have contemplated converting too, but eventually gave up the idea and remained an adherent of the Bosnian Church until his death.

Catherine proved to be a zealous convert. She initiated and financed from her dowry the construction of churches throughout Bosnia, starting with one in Kupres in 1447, followed by churches in Krupa na Vrbasu and Jezero; a church in Jajce was built in 1458. In December 1458 she wrote to Pope Pius II to request that the church in Jajce, built along with a Franciscan monastery and a vicariate, be named after her namesake, Saint Catherine of Alexandria. Pius conceded her request in a bull issued on 13 December, granting absolution to anyone who visited Queen Catherine's church at Christmas or Easter, or on certain feast days. Another church built by Queen Catherine in Jajce and dedicated to Saint Catherine was a simple royal chapel. The churches built in Vrbanja, Vranduk, Tešanj and Vrila are attributed to both Catherine and her husband.