Leguizamo will play Ernesto Clemente on the acclaimed NBC series. According to the Hollywood Reporter, his character is described as a "dynamic supervising physician who shakes up the County General emergency room with his unconventional ideas and use of cutting-edge technology."

On Sept. 22 "ER" will begin its 12th season on the peacock network. Leguizamo is slated to start his run Oct. 20. "ER" Executive Producer David Zabel told the industry paper, "John's combination of dramatic weight and comedic brilliance make him a vibrant, exciting addition to the cast."

The Colombian chameleon, who first caught the eyes of the theatre world with his one-man show Mambo Mouth, returned to Broadway with a limited engagement of his Tony-nominated show Sexaholix. The work, nominated for the Tony Award for Best Special Theatrical Event, played on stages across the country and was taped for HBO. The HBO version garnered Leguizamo an Emmy Award.

The actor received a Drama Desk Award for Best Solo Performance for Spic O-Rama. His stage work, Freak: A Semi Demi Quasi Pseudo Autobiography, garnered him a Tony nomination. Also a film star, he appeared as Toulouse Lautrec in Baz Luhrmann's "Moulin Rouge"; in "To Wong Foo, Thanks for Everything, Julie Newmar" in drag as Miss Chi-Chi Rodriguez; and in Luhrmann's "Romeo + Juliet" as a Virgin Mary gun-toting Tybalt. His biggest role came in the 1999 Spike Lee film "The Summer of Sam." Leguizamo also starred in the HBO drama "Undefeated," which he also directed, and will soon be seen in the independent film "Sueno."