Porto confirmed the signing on Wednesday, with the 36-year-old centre-back agreeing a contract until the end of the 2026/27 season. The deal includes an option for an additional year, which can be activated if specified sporting objectives are achieved.
Smalling had been a free agent since leaving Al-Fayha at the end of last season, following two seasons with the Saudi Arabian club.
According to Flashscore, Smalling underwent his medical and fitness tests in Porto before completing the move.
Smalling returns to European football
The move brings Smalling back to European football after his two-year spell in Saudi Arabia.
The defender began his senior career with Maidstone United before moving through Millwall and Fulham. His performances at Fulham earned him a move to Manchester United in 2010.
Smalling spent nine seasons at Old Trafford, making 324 appearances and scoring 18 goals. During his time with United, he won two Premier League titles, the Europa League, FA Cup, League Cup and three Community Shields.
He subsequently spent five seasons with Roma, where he won the inaugural UEFA Europa Conference League in 2022, before moving to Al-Fayha.
Porto add experience to defence
Smalling arrives at Porto with extensive experience at the highest level and 31 England caps to his name.
He represented England at the 2014 FIFA World Cup and UEFA Euro 2016. During his two seasons with Al-Fayha, he made 56 appearances.
Porto have brought him in to strengthen their defensive options as Francesco Farioli's side compete domestically and in European competition.
The Portuguese champions have handed Smalling the number six shirt as he begins his latest chapter in European football.
Smalling becomes Porto's latest experienced defensive addition and will now look to make an immediate impact with the Portuguese champions.