Boris Christoff (Bulgarian: Борис Кирилов Христов, official transliteration Boris Kirilov Hristov pronounced [bɔrˈis ˈkirilɔf ˈxristɔf]; 18 May 1914 – 28 June 1993) was a Bulgarian opera singer, widely considered one of the greatest basses of the 20th century.

Christoff had an excellent voice with a distinctive dark tone. Although it was not as large as some other bass voices, he had no trouble making an impact in big auditoria, like the San Francisco Opera. Owing to his stage presence and dramatic temperament, he was a worthy heir to the grand tradition of Slavonic basses exemplified by Fyodor Stravinsky, Lev Sibiriakov, Vladimir Kastorsky, Feodor Chaliapin, Alexander Kipnis and Mark Reizen, among others. He sang mostly in Verdi and the Russian repertoire, and was also a refined performer of vocal chamber music. Among his most famous roles were those of Tsar Boris (Mussorgsky - Boris Godunov), Philip II (Verdi - Don Carlo), Mephistopheles (Gounod - Faust and Boito - Mefistofele), Ivan Susanin (Glinka - A Life for the Tsar), Zaccaria (Verdi - Nabucco), Tsar Ivan (Rimsky-Korsakov - Ivan the Terrible), Dosifei (Mussorgsky - Khovanshchina), Gomez da Silva (Verdi - Ernani), Fiesco (Verdi - Simon Boccanegra), Attila (Verdi - Attila), Padre Guardiano (Verdi - La forza del destino), Galitzky and Kontchak (Borodin - Prince Igor) and others.

Christoff made studio recordings of eight operas (Don Carlo, Boris Godunov and Faust twice each) and numerous live recordings (radio or stage performances). He was much admired as song singer and he recorded more than 200 Russian songs by Mussorgsky (he was the first to record all his 63 songs), Tchaikovsky, Rimsky-Korsakov, Glinka, Borodin, Cui, Balakirev as well as traditional songs, mostly with piano accompaniment. He initiated the tradition of making studio recordings of Boris Godunov with the same basso singing three roles (Boris, Varlaam, Pimen).

While he was a grand performer on stage, Christoff had difficult off-stage relations with fellow singers and producers, which sometimes grew into public scandals. In 1955 he fell out with Maria Callas during the performances of Medea at the Rome Opera and in 1961 his contract with La Scala was terminated after an open conflict with fellow Bulgarian Nicolai Ghiaurov whom Christoff blamed for collaborating with the Bulgarian communist regime. Herbert von Karajan tried to make him sing the title role in Don Giovanni which would have been inappropriate for his range; this prompted him to sever relations with von Karajan.

He was the brother-in-law of the Italian baritone Tito Gobbi.