Terra Mariana (Land of Mary) is the official name for Medieval Livonia or Old Livonia (German: Alt-Livland, Estonian: Vana-Liivimaa, Latvian: Livonija), which is formed in the aftermath of the Livonian Crusade in the territories comprising present day Estonia and Latvia.

It is established on February 2, 1207, as a principality of the Holy Roman Empire but loses this status in 1215 when proclaimed by Pope Innocent III as a direct subject to the Holy See.

Terra Mariana is divided into feudal principalities by Papal Legate William of Modena: the Archbishopric of Riga, Bishopric of Courland, Bishopric of Dorpat, Bishopric of Ösel-Wiek, the lands ruled by the Livonian Brothers of the Sword.

The northern parts become a Dominum directum to the King of Denmark, Duchy of Estonia.

After the 1236 Battle of Saule, the surviving members of the brothers merge in 1237 with the Teutonic Order of Prussia and become known as the Livonian Order.

In 1346 the order buys Danish Estonia.

Throughout the existence of medieval Livonia there is a constant struggle over the supremacy of ruling the lands 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 Ordenstaat fall into decline but the Livonian Order manages to maintain its independent existence.

In 1561, during the Livonian war, Terra Mariana ceases to exist.

Its northern parts are ceded to Sweden and formed into the Duchy of Estonia, its southern territories become part of the Grand Duchy of Lithuania — and thus eventually of the Polish–Lithuanian Commonwealth as the Duchy of Livonia and Duchy of Courland and Semigallia.

The island of Saaremaa becomes part of Denmark.