The right back will arrive for a £11 million fee on deadline day.
However, Porro is going straight back out on loan to Real Valladolid.
The youngster, who is only 19 and can play all along the right flank, will be hoping to gain first team experience with the Spanish club.
Porro played 35 games for Girona, managing a goal and four assists in those appearances.
Girona's sporting director Quique Carcel spoke about the outgoing transfer and said: "In these two years in Girona, Pedro has had a very important evolution and because of this we knew that this summer a transfer could happen.
"Being so young and playing in the Second Division could reduce his value and that is why we have opened the market."