César Antonovitch Cui (Russian: Цезарь Антонович Кюи) (January 6/18, 1835 – March 13, 1918) was a Russian composer and music critic of French and Lithuanian descent. He was a member of The Five, the group of Russian composers under the leadership of Mily Balakirev dedicated to the production of a specifically Russian brand music.

Cui was born in Vilnius, now in Lithuania. He received piano lessons and instruction in music theory as a child, but after leaving school he entered Saint Petersburg's School of Military Engineering, and began a military career. He became an expert on fortifications. It was in 1857, when Cui met Mily Balakirev, that he became more seriously involved with music, becoming a member of what would eventually be constituted and known as The Five.

Even though he was composing music and writing music criticism in his spare time, Cui turned out to be an extremely prolific composer and writer. In addition to many piano and chamber pieces, numerous choruses, several orchestral works, and hundreds of songs, he seems to have put his greatest hope in his operas, of which he composed fifteen. In 1869 the first public performance of an opera by Cui took place; this was his William Ratcliff (based on the tragedy by Heinrich Heine); but it did not have success after eight performances, partially because of the harshness of his own writings in the music press. All but one of his operas were composed to Russian texts; the one exception, Le Flibustier (on a play by Jean Richepin), premiered at the Opera-Comique in Paris in 1894, but was not a success, either, primarily because of the vogue for Richard Wagner. Cui's more successful stage works during his lifetime were the one-act comic opera The Mandarin's Son (publically premiered in 1878), the three-act Prisoner of the Caucasus (1883), based on Pushkin, and the one-act Mademoiselle Fifi (1903), based on Maupassant.

As a writer on music, Cui contributed almost 800 articles between 1864 and 1918 (mostly during 1864-1900) to various newspapers and other publications in Russia and Europe. Several of his themed sets of articles were reissued as monographs: Kol'tso Nibelungov (The Nibelung Ring, 1876); La musique en Russie (1880); Russkii romans (The Russian Romance, 1896).

As to Cui's current status, in the last few decades one of his children's opera (of which he composed four) entitled Puss-in-Boots (from Perrault) has had wide appeal in Germany. Nevertheless, despite the fact that more and more of Cui's music is being made available in recent years in recordings (including his short opera Feast in Time of Plague, from Pushkin), his status today in the repertoire is considerably small, based (in the West) primarily on some of his piano and chamber music (such as the violin and piano piece called Orientale (op. 50, No. 9)) and a number of solo songs. The received wisdom that he is not a particularly talented composer, at least for large genres, has been cited as a cause for this state of affairs.

Cui's works are not so nationalistic as those of the other members of The Five; with the exception of Pushkin, his operas do not display a strong attraction to Russian sources, but he did write innumerable songs and choruses to texts by Russian composers. His style is more often compared to Robert Schumann and to French composers than to Mikhail Glinka or to Cui's Russian contemporaries. However, his work as a critic did help to promote the works of the other, now better remembered, members.

Cui died on March 13, 1918 and was buried next to his wife Mal'vina, who had died in 1899, at the Lutheran Semetary in Smolensk. In 1939 his body was reinterred in Tikhvin Cemetery at the Alexander Nevsky Monastery, St. Petersburg, Russia to lie beside the other members of The Five.

The most recent full-length biography of Cui is A.F. Nazarov's Tsezar' Antonovich Kiui (Moskva: Muzyka, 1989).