Smalling, who has won 31 caps for England, spent two seasons with Al Fayha and is now set to join Porto without a fee.
According to Maisfutebol, the 36-year-old is already in Porto, where he will undergo medical and fitness tests before formally signing a contract.
ScoresStat understands that the deal will run until the end of the season, with an option for a further year. It is expected to be signed later on Wednesday.
Developed at Millwall and Maidstone United, Smalling stepped up to Fulham in 2008/09 before moving to Manchester United, where he spent the period between 2010 and 2019.
Before joining Al Fayha, the centre-back also represented Roma from 2019 to 2024.
Smalling joins Porto to provide greater competition for Jan Bednarek and Jakub Kiwior, following the arrival of experienced Brazilian defender Thiago Silva last season.
Alongside the Polish duo, Francesco Farioli also has Nehuen Perez, who returned from a lengthy lay-off but suffered another injury, and Dominik Prpic, a Croatian centre-back who has been featuring for the second team, as options in central defence.
