State television showed images of Mr Castro at a meeting with cheese experts near the capital, Havana.
Fidel Castro, 88, handed power to his younger brother Raul in 2006 after struggling to recover from illness.
His appearance comes days after Cuba and the United States announced they were reopening embassies in each other's capitals on 20 July.
Mr Castro is rarely seen in public, and secrecy about his health means his public appearances are scrutinised by the media.
The historic thaw between the old Cold War enemies was announced on 17 December, in joint media conferences by Presidents Barack Obama and Raul Castro.
There was no immediate comment from Fidel Castro, and analysts said the rapprochement would not have happened if he was still in power.
Just over a month later, he broke his silence in a letter published on the Cuban Communist Party newspaper, Granma.
"I don't trust the policy of the United Statesâ€¦ but this does not mean I reject a pacific solution to the conflicts," he wrote.
Relations had been frozen since the early 1960s when the US broke links and imposed a trade embargo on Cuba.
Former Cuban leader Fidel Castro has appeared in public for the first time in more than a month.