Theodore Kosloff was a Russian-born ballet dancer, choreographer and film and stage actor. He was occasionally credited as Theodor Kosloff.
Born Fyodor Mikhailovich Koslov in Moscow in 1882, Kosloff began his professional ballet career after training at Moscow's Imperial Theater. After graduating in 1901, he began touring internationally with the Diaghilev Ballet Company which he had joined in 1909. While touring with the company, Kosloff began a romantic relationship with fellow company member, the American future set-designer and Mrs. Rudolph Valentino, Natacha Rambova. The affair however, was brief and allegedly tumultuous.
After arriving in the United States of America in 1909, Kosloff was introduced to influential film director Cecil B. DeMille by the actress and writer Jeanie MacPherson. DeMille was also encouraged to sign Kosloff due to the persistence of his young niece Agnes de Mille, who was an ardent fan of the ballet dancer. DeMille was immediately impressed by the dark-haired young dancer and quickly put Kosloff to work as an actor. Kosloff's first role was in the 1917 DeMille directed The Woman God Forgot opposite the extremely popular American singer and actress, Geraldine Farrar.
Kosloff also worked steadily during his acting career as a choreographer and between 1912 and 1916 choreographed several Broadway musicals: The Passing Show of 1915, A World of Pleasure and See America First. From 1918 through 1919 Kosloff also appeared on the stage as an actor in the revival of The Awakening.