The Diploma Ottonianum (also called the Pactum Ottonianum, Privilegium Ottonianum or simply Ottonianum) was an agreement between Pope John XII and Otto I, King of Germany and Italy. It confirmed the earlier Donation of Pepin, granting control of the Papal States to the Popes, regularizing Papal elections, and clarifying the relationship between the Popes and the Holy Roman Emperors.

Otto I (23 November 912 – 7 May 973), traditionally known as Otto the Great (German: Otto der Große, Italian: Ottone il Grande), was East Francian king from 936 and Holy Roman Emperor from 962 until his death in 973. He was the oldest son of Henry I the Fowler and Matilda.

Otto inherited the Duchy of Saxony and the kingship of the Germans upon his father's death in 936. He continued his father's work of unifying all German tribes into a single kingdom and greatly expanded the king's powers at the expense of the aristocracy. Through strategic marriages and personal appointments, Otto installed members of his family in the kingdom's most important duchies. This reduced the various dukes, who had previously been co-equals with the king, to royal subjects under his authority. Otto transformed the church in Germany to strengthen royal authority and subjected its clergy to his personal control.

After putting down a brief civil war among the rebellious duchies, Otto defeated the Magyars at the Battle of Lechfeld in 955, thus ending the Hungarian invasions of Western Europe. The victory against the pagan Magyars earned Otto a reputation as a savior of Christendom and secured his hold over the kingdom. By 961, Otto had conquered the Kingdom of Italy. The patronage of Otto and his immediate successors facilitated a so-called "Ottonian Renaissance" of arts and architecture. Following the example of Charlemagne's coronation as "Emperor of the Romans" in 800, Otto was crowned Holy Roman Emperor in 962 by Pope John XII in Rome.

Otto's later years were marked by conflicts with the papacy and struggles to stabilize his rule over Italy. Reigning from Rome, Otto sought to improve relations with the Byzantine Empire, which opposed his claim to emperorship and his realm's further expansion to the south. To resolve this conflict, the Byzantine princess Theophanu married his son Otto II in April 972. Otto finally returned to Germany in August 972 and died at Memleben in May 973. Otto II succeeded him as Holy Roman Emperor.

Otto has been consistently depicted in historiography through different eras as a successful ruler. Modern historians, while not denying his strong character and his many fruitful initiatives, explore the emperor's capability as a consensus builder – a process that goes in parallel with greater recognition of the nature of consensus politics in Medieval Europe (especially Western and Central parts) as well as different roles played by other actors in his time. As one of the most notable Holy Roman Emperors, Otto's footprint in artistic depictions is also considerable.