Rubén Castillo (born August 12, 1954) is a former United States district judge of the United States District Court for the Northern District of Illinois. In 1994, Castillo was the first person of Latino descent to be named a judge in the Northern District of Illinois, and in 2013 the first to become Chief Judge. 

Born in Chicago, Illinois to a Mexican-American father and a Puerto Rican mother,[2] Castillo was the first member of his family to finish college.[3] Castillo earned a Bachelor of Arts degree in political science in 1976 from Loyola University in Chicago, working nights as a clerk at the Illinois Circuit Court of Cook County to put himself through school.[3] He then earned a Juris Doctor from Northwestern University School of Law in 1979.

From 1979 until 1984, Castillo worked in private law practice in Chicago as an associate attorney for the law firm Jenner & Block.[5] In 1984, he was named an Assistant United States Attorney for the Northern District of Illinois. He worked as an Assistant United States Attorney until 1988, when he became a regional counsel for the Mexican American Legal Defense Fund. In 1991, Castillo returned to private law practice, where he worked as a partner at the law firm of Kirkland & Ellis until 1994.[5][4] Following his retirement from the judiciary, Castillo rejoined private practice in the Chicago office of Akerman LLP.