The Puerto Rico-born actor interrupted his vacation in the U.S. Caribbean territory for the talk at Bayamon Regional Prison outside the capital, San Juan.
"I believe everyone needs a second chance," Del Toro told about 30 inmates who take part in the theater group. "I am here to help reaffirm that as long as there is life, you can still help society in some way, you can improve yourself."
He encouraged the inmates to read as part of their rehabilitation, saying reading has been crucial to his own growth as an actor.
The inmates performed a sketch for the actor and thanked him for coming.
"It's a great thing to be able to share with an artist who is famous, who has been making movies in Hollywood," said Carlos Baez Figueroa, 39, who is in prison for murder.
Del Toro won a supporting actor Oscar for his role in the movie "Traffic" and starred in last year's "Che," a film biography of Latin American revolutionary Che Guevara.
Related Content from AccessHollywood.com:
VIEW THE PHOTOS: Oscars 2009: Sharp-Looking Men
PLAY IT NOW: Access Extended: 'Thing We Lost In The Fire' Premiere
PLAY IT NOW: Halle Berry Talks 'Things We Lost In The Fire'