Argentina international Pastore, a big money arrival from Palermo this summer, slotted home the only goal of the game after 68 minutes to give his side the points.
After a slow start to their campaign, which began with a draw and a defeat, Antoine Kombouare's side are now heading in the right direction and are up to fourth in the standings.
The game started lethargically, with neither side able to stamp their authority on proceedings and no chances of note until the 19th minute.
Brest set up a quick counter attack, with Nolan Roux finding Romain Poyet just outside the area as the move gathered pace.
Poyet managed to get a shot on goal away but Salvatore Sirigu was able to get down low to make the save.
PSG hit back through Pastore four minutes later.
He collected the ball on the edge of the penalty box, feinted one and then went the other before firing at the top corner where Steve Elana made the save.
Brest were finding their feet, though, Poyet's cross requiring the urgent attention of Sirigu, whose parry offered Benoit Lesomier a chance.
He shot over and there was an even easier chances wasted soon after.
Roux eased past his man on the right and crossed for Eden Ben Basat, who somehow managed to miss the target when a goal seemed certain.
Pastore gave one more warning of his talent before half-time, beating three men before playing in Mathieu Bodmer, who missed the target.
PSG needed to improve after the break and did, but only just enough to edge an uninspiring 45 minutes.
Five minutes after the resumption, Pastore teed up Kevin Gameiro but he found the angle too tight.
Brest's energy was now fading and they could have conceded when Siaka Tiene's cross was headed unconvincingly at Elana.
Pastore got it right 22 minutes from time, Jeremy Menez the creator as he dribbled past a series of Brest defenders before finding Pastore.
He produced a clinical right-footed finish, which was enough to close out the result.