The Cuban leader expressed "admiration" for the "extraordinary battle" waged by Maduro, the handpicked successor of late president Hugo Chavez, who now will be forced to work with an opposition-dominated Congress.
"Dear Maduro: I have followed minute by minute, the extraordinary battle that has been been waged," Castro wrote in the state-run Granma newspaper.
"I am certain that new victories will come to the Bolivarian Revolution," the Cuban president wrote, referring to the socialist policies put in place by the firebrand Chavez.
Maduro promptly accepted the defeat, a blow to his leadership and the "revolution" of "21st century socialism" launched under Chavez.