The Portugal midfielder joined the Ligue 1 champions on transfer deadline day in a temporary deal until June 2021, with PSG retaining an option to buy the 29-year-old permanently.
Pereira joined Porto from Maritimo in 2015 and he has gone on to make 202 appearances for the club, winning the Primeira Liga in 2018 and last season.
He has also featured in 30 Champions League matches, scoring once and laying on three assists.
"It's a new challenge for me and to be part of Paris Saint-Germain fits my ambitions," said Pereira.
"Joining one of the biggest clubs in Europe and in the world is a huge moment in my career and I hope to be able to bring a lot to Paris and its supporters."
The arrival of Pereira comes a day after PSG signed striker Moise Kean on loan from Everton.
PSG boss Thomas Tuchel last week warned the club would not be able to be such a force in the Champions League unless they strengthen before the window closes.
That led to a retort from sporting director Leonardo, who said: "We did not like the declaration. If someone is not happy, we talk. But if he decides to stay, he must respect internal rules."