The Pactum Lotharii was an agreement signed on 23 February 840, between Republic of Venice and the Carolingian Empire, during the respective governments of Pietro Tradonico and Lothair I. This document was one of the first acts to testify to the separation between the nascent Republic of Venice and the Byzantine Empire: for the first time the Doge, on his own initiative, undertook agreements with the Western world.

The treaty included a commitment on the part of the Venetians to help the empire in its campaign against the Slavic tribes. In return, it guaranteed Venice's neutrality as well as its security from the mainland. However, the treaty did not end the Slavic plunderings since by 846, the Slavs were still recorded menacing cities such as the fortress of Carolea.

In the pactum Lotharii, Venice promised not to buy Christian slaves in the Empire, and not to sell Christian slaves to Muslims. The Venetians subsequentently began to sell Slavs and other Eastern European non-Christian slaves in greater numbers. Caravans of slaves traveled from Eastern Europe, through Alpine passes in Austria, to reach Venice. Surviving records valued female slaves at a tremissa (about 1.5 grams of gold or roughly 1⁄3 of a dinar) and male slaves, who were more numerous, at a saiga (which is much less). Eunuchs were especially valuable, and "castration houses" arose in Venice, as well as other prominent slave markets, to meet this demand.