But Castro's thumbs-up may be the last thing Obama wanted, as his words will only strengthen conservative complaints that the prize was an anti-American gesture.
"Many believe that he still has not earned the right to receive such a distinction. But we would like to see, more than a prize for the US president, a criticism of the genocidal policies that have been followed by more than a few presidents of that country," The Telegraph quoted Castro, 83, as saying.
Castro, in his regular 'Reflections of Comrade Fidel,' wrote that he had often disagreed with the choice of Norway's Nobel judges, but this time, "I must admit that in this case, in my opinion, it was a positive step".
Castro has praised some of the young American president's policies in recent months, while criticising him for not lifting the US trade embargo on the Caribbean island.
"This is ridiculous-embarrassing, even...I admire President Obama. I like President Obama. I voted for President Obama. But the peace prize?" wrote Ruth Marcus of the Washington Post.
"The Nobel Peace Prize award to Barack Obama seems so goofy, even if you're a fan, you have to admit that he hasn't really done much yet as a peacemaker," wrote another columnist David Ignatius.