Alexander Alexandrovich Vasiliev (Russian: Алекса́ндр Алекса́ндрович Васи́льев; 4 October 1867 – 30 March 1953) was considered the foremost authority on Byzantine history and culture in the mid-20th century. His History of the Byzantine Empire (vol. 1–2, 1928) remains one of a few comprehensive accounts of the entire Byzantine history, on the par with those authored by Edward Gibbon and Fyodor Uspensky.
Vasiliev studied under one of the earliest professional Byzantinists, Vasily Vasilievsky, at the University of St Petersburg and later taught Arabic language there. Between 1897 and 1900, he furthered his education in Paris. In 1902, he accompanied Nicholas Marr in his trip to Saint Catherine's Monastery in Sinai.
During his stay at the Tartu University (1904–12), Vasiliev prepared and published a highly influential monograph, Byzantium and the Arabs (1907). He also worked in the Russian Archaeology Institute, established by Fyodor Uspensky in Constantinople. In 1912, he moved to the St Petersburg University as a professor. He was elected to the Russian Academy of Sciences in 1919.
In 1925, during his visit to Paris, Vasiliev was persuaded by Michael Rostovtzeff to emigrate to the West. It was Rostovtzeff who ensured a position at the University of Wisconsin–Madison for him. Several decades later, Vasiliev moved to work in Dumbarton Oaks. Towards the end of his life, he was elected President of the Nikodim Kondakov Institute in Prague and of the Association Internationale des Études Byzantines.