Otamendi, 32, wore the Porto shirt between 2010 and 2014, winning three league titles and the Europa League.
Since his move from Valencia to Manchester City in 2015, he has found more success as part of Pep Guardiola’s side.
Otamendi has won nine trophies in the sky blue of City, including three Premier League titles.
He was named in the 2017-18 PFA Team of the Year following an excellent season with City.
Porto make contact
However, Guardiola no longer counts on Otamendi like before and the Argentine can leave City if a good offer is made.
O JOGO reports that Porto boss Sergio Conceicao wants another central defender and Otamendi is his choice.
Porto have initiated contacts to explore a return to the club, with agent Jorge Mendes heavily involved.
For Otamendi to join Porto, he would have to accept a significant salary reduction on what he earns at City.
Otamendi, who is also on Real Betis’ radar, still has two years left on his Man City contract.