The 37-year-old was taken to hospital, the Portuguese club said in a statement, adding that the goalkeeper’s life was not in danger.
“Iker Casillas suffered an acute heart attack during this Wednesday’s training session,” Porto said in a statement on their official website.
“The session was quickly halted so medical assistance could be provided to the Porto goalkeeper, who is currently in the CUF Porto Hospital. Casillas is OK, stable, and the heart problem has been resolved.”
The club’s official Twitter account added: “Always together. Stay strong, Iker” and the club’s players and staff recorded a message of support for their veteran goalkeeper.