César Cui (January 18 (J: January 6) 1835 Vilnius, Russian Empire – March 13, 1918 Saint Petersburg, Russian Soviet Republic) was a Russian composer and music critic. He was a member of the nationalist Group of Five led by Mili Balakirev.

Cui was of French-Lithuanian descent. He was born in Vilnius, the present-day capital of Lithuania, received music lessons as a child, and began his military career in Saint Petersburg. He became an expert on fortifications. In 1857, Cui met Balakirev and began to write and criticize music more seriously in his spare time. Cui proved to be a very prolific composer: in addition to numerous piano and chamber works, he composed fifteen operas, several choral and orchestral works, and hundreds of songs.