Justino Díaz (born January 29, 1940) is a Puerto Rican operatic bass-baritone. In 1963, Díaz won an annual contest held at the Metropolitan Opera of New York, becoming the first Puerto Rican to obtain such an honor and as a consequence, made his Metropolitan debut in October 1963 in Verdi's Rigoletto as Monterone.

Díaz was born in San Juan, Puerto Rico, the capital of the island, but lived and was raised in the town of Cataño. He attended Robinson Elementary School where, at the age of 8, he started to participate in the school's activities as a singer. In his first school play, when he was 10 years old, he sang the song "Old Black Joe", which became his favorite.