Terra Mariana (Medieval Latin for "Land of Mary") was the official name for Medieval Livonia or Old Livonia (Low German: Oolt-Livland, Livonian: Jemā-Līvõmō, Estonian: Vana-Liivimaa, Latvian: Livonija). It was formed in the aftermath of the Livonian Crusade, and its territories were composed of present-day Estonia and Latvia. It was established on 2 February 1207, as a principality of the Holy Roman Empire, but lost this status in 1215 when Pope Innocent III proclaimed it as directly subject to the Holy See.The papal legate William of Modena divided Terra Mariana into feudal principalities: the Duchy of Estonia (dominum directum to the king of Denmark); the Archbishopric of Riga; the Bishopric of Courland; the Bishopric of Dorpat; the Bishopric of Ösel–Wiek; and territories under the military administration of the Livonian Brothers of the Sword. After the 1236 Battle of Saule, the surviving members of the Brothers merged in 1237 with the Teutonic Order of Prussia and became known as the Livonian Order. In 1346 the Livonian Order bought the Duchy of Estonia from Denmark.

Throughout the existence of medieval Livonia there was a constant struggle over supremacy, between the lands ruled by the Church, the Order, the secular German nobility, and the citizens of the Hanseatic towns of Riga and Reval. Following its defeat in the Battle of Grunwald in 1410, the Teutonic Order and the State of the Teutonic Order fell into decline, but the Livonian Order managed to maintain its independent existence.

In 1561, during the Livonian War, Terra Mariana ceased to exist. Its northern parts were ceded to Sweden and formed into the Duchy of Estonia, its southern territories became part of the Grand Duchy of Lithuania — and thus eventually of the Polish–Lithuanian Commonwealth — as the Duchy of Livonia and the Duchy of Courland and Semigallia. The island of Saaremaa became part of Denmark. Since the beginning of the 20th century Terra Mariana (Estonian: Maarjamaa) has been used as a poetic name or sobriquet for Estonia. In 1995 the Order of the Cross of Terra Mariana, a state decoration, was instituted to honor the independence of Estonia.