Today's gateway is the renovation of a wall built in the 17th century by Pope Urban VIII who, in response to the disastrous sacking of Rome by German mercenaries, wanted to fortify the Janiculum Hill. The present gateway takes the place of the earlier Porta Portuensis:

At the end of the 3rd century AD, for the first time ever, the Emperor Aurelianus decided to enclose Trastevere and the Janiculum Hill inside the walls of the city because he considered these districts essential to Rome's defenses. The Via Portuensis, like so many other Roman roads, took its name from its destination.
It left from this gate for the ports built by Claudius and Trajan. It was therefore one of the most important commercial arteries as it was used for supplying Rome's markets.
Porta Portese, even today, hosts Rome's flea market which takes place each Sunday morning and extends all the way up to Viale Trastevere.