CÚsar Antonovitch Cui (January 18, 1835 - March 26, 1918) was a Russian composer and music critic of French descent. He was a member of The Five, the group under the leadership of Mily Balakirev dedicated to the production of a specifically Russian 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, composing music in his spare time. It was in 1857, when Cui met Mily Balakirev, that he became more interested in music, becoming a member of what is now known as The Five.

Cui was an extremely prolific composer, and in 1869 had some public success with his opera William Ratcliff (based on the tragedy by Heinrich Heine). In 1884 Le Flibustier was also well received. However, with the exception of one piece for violin and piano called Orientale, none of his pieces are often performed today, and he is not regarded as a particularly talented composer.

Cui's works are not so nationalistic as those of the other members of The Five - he was not so attached to Russian themes in his operas, and his style is more often compared to Robert Schumann than Mikhail Glinka or his Russian contemporaries. However, his work as a critic did help to promote the works of the other, now better remembered, members.

CÚsar Cui died on March 26, 1918 and was interred in Tikhvin Cemetery at the Alexander Nevsky Monastery, St. Petersburg, Russia.