The 81-year-old had said he was not well enough to continue in his role his Cuban president after falling ill in 2006.
Many people expect Fidel's brother Raul, who is five years younger and has been acting as president, to step into the role at a National Assembly meeting on Sunday.
Castro said he had planned on taking a break from his newspaper columns for at least 10 days, but decided: "I didn't have the right to keep silent for so long."
Castro also used the column, titled "Reflections of Comrade Fidel," to poke fun at the US presidential candidates, saying his retirement had forced them to talk about Cuba.
"I enjoyed observing the embarrassing position of all the presidential candidates in the United States," he wrote.
"One by one, they could be seen forced to proclaim their immediate demands to Cuba so as not to alienate a single voter."
He also criticised demands by the candidates and by George Bush, the US president, for political change on the island.
"'Change, change, change!' they shouted in unison. I agree. 'Change!' But in the United States,'' he wrote.
"Cuba changed a while ago and will continue on its dialectical course."
Castro asked the Cuban press authorities to publish the column on page four of the newspaper instead of the front page, where his previous columns had been published.