Casillas, 39, suffered a heart attack during a training session last year and has not played any competitive football since April, 2019.
He was given a role on the team's technical staff at the start of this season and was included in the first-team squad, although he was never named in a matchday squad.
Porto president Jorge Nuno Pinto da Costa said in February that Casillas was set to retire from playing in order to stand for election to become president of the Spanish soccer federation, although he later withdrew from the running due to the COVID-19 pandemic.
Casillas, who led Spain to their first World Cup in 2010 and helped his country win the 2008 and 2012 European championships, had a low-key farewell from the club he joined in 2015 after spending 25 years at Real Madrid.
He posted a picture on his official Twitter account from Monday's 1-0 win at Pacos Ferreira with the caption "the last trip".
Porto released a video on their website titled "Always one of us" in tribute to the goalkeeper, who made 156 competitive appearances for the club and won the 2017-18 league title with them.