"I would give it to Casillas," Rajoy said when asked who he would support if he could vote for FIFA's Ballon d'Or.
The prime minister recalled Casillas's role in Spain's World Cup victory and consecutive Euro wins, and his role in Real Madrid's past Champions League success.
"Being a goalkeeper is very difficult," Rajoy told Cope radio.
The Spanish leader praised other contenders this year such as the favourite Barcelona striker Lionel Messi and Real Madrid's forward Cristiano Ronaldo and its central defender Sergio Ramos.
"But if I had to vote, I would vote Casillas," he said.
Casillas has cast doubt over whether any goalkeeper can win, telling France Football magazine that despite being the only indispensible player in a team, a keeper just doesn't have the glamour of the outfield players.
Spain's Prime Minister backs Iker Casillas for Ballon d'Or
Advertisement