Count Aleksey Konstantinovich Tolstoy (Russian: Граф Алексе́й Константи́нович Толсто́й; September 5 [O.S. August 24] 1817 – October 10 [O.S. September 28] 1875), often referred to as A. K. Tolstoy, was a Russian poet, novelist, and playwright. He is considered to be the most important nineteenth-century Russian historical dramatist, primarily on account of the strength of his dramatic trilogy The Death of Ivan the Terrible (1866), Tsar Fyodor Ioannovich (1868), and Tsar Boris (1870). He also gained fame for his satirical works, published under his own name (History of the Russian State from Gostomysl to Timashev, The Dream of Councillor Popov) and under the collaborational pen name of Kozma Prutkov. His fictional works include the novella The Family of the Vourdalak, The Vampire (1841), and the historical novel Prince Serebrenni (1862).

Aleksey was a member of the Tolstoy family, and a second cousin of Leo Tolstoy. Due to his mother's closeness with the court of the Tsar, Aleksey was admitted to the future Alexander II's childhood entourage and became "a comrade in games" for the young Crown Prince. As a young man Tolstoy traveled widely, including trips to Italy and Germany, where he met Johann Wolfgang von Goethe. Tolstoy began his education at home under the tutelage of his uncle the writer Antony Pogorelsky, under whose influence he first became interested in writing poetry, and a number of other teachers. In 1834 Tolstoy enrolled in the Moscow Foreign Ministry State Archive as a student. In December 1835 he completed exams (in the English, French, and German languages as well as in literature, Latin, World and Russian history, and Russian statistics) at the University of Moscow.

Throughout the 1840s, Tolstoy led a busy high society life, full of pleasure trips, salon parties and balls, hunting sprees, and fleeting romances. He also spent many years in state service as a bureaucrat and diplomat. In 1856, on the day of his Coronation, Alexander II appointed Tolstoy one of his personal aide-de-adjutants. Tolstoy served as an infantry major in the Crimean War. He eventually left state service in the early 1860s to pursue his literary career. He died in 1875 of a self-administered lethal dose of morphine at his Krasny Rog estate in the Chernigov Governorate.