Writing about the US presidential race on the website Cubadebate, ahead of the November 4 election, he said: "Profound racism exists in the United State,
"Millions of whites cannot reconcile in their minds with the idea that a black man with his wife and children would move into the White House, which is called just like that – White," he wrote.
He added that it was a "pure miracle" the Illinois senator, who became the first black politician to win the Democratic Party's presidential nomination, had not been assassinated.
The ailing Cuban communist leader, who handed power to his brother Raul earlier this year, described Mr Obama as a politician who "has the habit of looking at his opponent with serenity and laughing at his rhetorical attacks."
Mr Castro's comments came as a war of words with racial undertones marked the US White House race after civil rights icon John Lewis accused Republican candidate John McCain of sowing "hatred" against Mr Obama, who is developing a commanding lead in opinion polls.
Mr Castro also mocked Mr McCain's poor grades at the naval academy and said Republican vice presidential nominee Sarah Palin "knew nothing at all about anything."