Casillas spent five days in hospital after suffering an ‘acute myocardial infarction’ whiles training with his teammates in May and he played no part in FC Porto’s final few games of the season.
Casillas, who has one more year left on his contract at the Estadio do Dragao, made 31 Primeira Liga appearances as FC Porto finished second in the league behind Benfica.
Casillas was present as Porto began preparations for next season and he tweeted his arrival at the club’s training complex with the words: "Back to work. Day one."
Reports suggested Casillas was set to retire from the sport as a result of his setback but he came out to deny those rumors after receiving positive news from his doctors.
Casillas, 38, is widely regarded as one of the greatest goalkeepers of his generation.
He joined FC Porto from Real Madrid in 2015 and has won one Portuguese League title.
In 16 years at Real Madrid between 1999 and 2015, Casillas won three UEFA Champions League trophies, five Spanish La Liga titles and two Copa del Reys, amongst numerous other trophies.
Casillas, who holds the record for most appearances in the history of the UEFA Champions League (177), captained Spain to win the European Championships in 2008 and 2012 as well as the FIFA World Cup in 2010.
