The former Spain and Real Madrid goalkeeper, 38, was included in the Porto's squad list on the league's official website ahead of their opening league game away to Gil Vicente on Saturday.
Casillas, who captained Spain to their only World Cup triumph in 2010 and played 725 games for Real Madrid before joining Porto in 2015, suffered a heart attack in a training session on May 1 and spent five days in hospital.
Porto president Jorge Nuno Pinto da Costa did not expect Casillas to play again and began to search for his replacement shortly after the goalkeeper's heart attack.
However, in July, Porto said Casillas had been given a role on the team's technical staff "while he recovers from his health problem."
Porto signed Argentine goalkeeper Agustin Marchesin from Mexican side Club America in July, and the 31-year-old made his debut on Wednesday in the 1-0 win away to Krasnodar in a Champions League qualifying match.