The Williams driver beat Red Bull's Mark Webber and Ferrari's Felipe Massa to the fastest time during a late scramble on soft tyres.
The result means that Rosberg has now topped five out of the six free practice sessions held so far in 2009, with only Ferrari's one-two yesterday afternoon interrupting his streak.
The two Brawns ended up only ninth and 10th.
The session got off to a hesitant start as the teams waited for some spots of moisture to clear. The clouds remained dark and very ominous for most of the hour, but the rain held off and the track stayed dry.
Jenson Button led the way for Brawn once the first runs were complete, before Rosberg moved ahead after 26 minutes.
Sebastian Vettel briefly usurped the Williams with his Red Bull one minute later, but Rosberg was already responding and soon regained the top spot by 0.245 seconds.
With the clouds still threatening, teams switched to soft tyres much earlier than usual. BMW's Nick Heidfeld was the first to try the faster rubber, lapping in 1m37.026s to beat Rosberg by 0.4s with 23 minutes still on the clock.
Brawn GP then reasserted its authority, with Button out-pacing Heidfeld by half a second on his own first soft tyre lap.
Barrichello also took a turn in front - despite a huge puff of smoke from the back of his car during his hot lap - but having completed their qualifying simulations early, the two Brawns were pushed right back to the foot of the top ten when others improved.
As the whole field switched to softs, Ferrari briefly held a one-two with first Kimi Raikkonen, then Massa going quickest.
But with four minutes to go, Rosberg moved back to his habitual position in front with a lap of 1m35.940s.
Webber grabbed second in the closing minutes, ahead of Massa and the two Toyotas, with Vettel ending up sixth.
Raikkonen eventually fell to seventh, followed by Williams's Kazuki Nakajima.
Another difficult session for McLaren saw Lewis Hamilton and Heikki Kovalainen back in 12th and 13th, with Fernando Alonso also seemingly struggling back in 14th.
Pos Driver Car Time Laps
1. Rosberg Williams-Toyota (B) 1:35.940 19
2. Webber Red Bull-Renault (B) 1:36.048 + 0.108 13
3. Massa Ferrari (B) 1:36.089 + 0.149 13
4. Trulli Toyota (B) 1:36.132 + 0.192 21
5. Glock Toyota (B) 1:36.189 + 0.249 22
6. Vettel Red Bull-Renault (B) 1:36.194 + 0.254 14
7. Raikkonen Ferrari (B) 1:36.322 + 0.382 14
8. Nakajima Williams-Toyota (B) 1:36.325 + 0.385 18
9. Barrichello Brawn-Mercedes (B) 1:36.519 + 0.579 19
10. Button Brawn-Mercedes (B) 1:36.541 + 0.601 17
11. Kubica BMW-Sauber (B) 1:36.563 + 0.623 18
12. Hamilton McLaren-Mercedes (B) 1:36.657 + 0.717 15
13. Kovalainen McLaren-Mercedes (B) 1:36.742 + 0.802 13
14. Alonso Renault (B) 1:37.004 + 1.064 16
15. Heidfeld BMW-Sauber (B) 1:37.026 + 1.086 18
16. Piquet Renault (B) 1:37.032 + 1.092 18
17. Sutil Force India-Mercedes (B) 1:37.118 + 1.178 18
18. Buemi Toro Rosso-Ferrari (B) 1:37.282 + 1.342 17
19. Bourdais Toro Rosso-Ferrari (B) 1:37.322 + 1.382 16
20. Fisichella Force India-Mercedes (B) 1:37.398 + 1.458 19