Modest Ilyich Tchaikovsky (13 May [O.S. 1 May] 1850–15 January [O.S. 2 January] 1916) was a Russian dramatist, opera librettist and translator.
Modest Ilyich was born in Alapayevsk, Verkhotursky Uyezd, Perm Governorate, the younger brother of the composer Pyotr Ilyich Tchaikovsky. He graduated from the Imperial School of Jurisprudence with a degree in law. In 1876, Modest became the tutor to a deaf-mute boy Nikolai ("Kolya") Hermanovich Konradi (1868–1922) and, using a special teaching method, helped him to talk, write, and read. In his still unpublished Autobiography, broadly quoted by Alexander Poznansky, Modest Ilyich Tchaikovsky mentions his homosexuality, as well as his brother Pyotr Ilyich Tchaikovsky's homosexuality.
Modest chose to dedicate his entire life to literature and music. He wrote plays, translated sonnets by Shakespeare into Russian and wrote librettos for operas by his brother Pyotr, as well as for other composers such as Eduard Nápravník, Arseny Koreshchenko, Anton Arensky and Sergei Rachmaninoff. Being the nearest friend of his brother, he became his first biographer, and also the founder of the Tchaikovsky Museum in Klin.