The Spaniard, who also won on home turf in Valencia in June, dominated the mandatory pitstop race from pole position.
He moved over on fellow front-row starter Andreas Zuber at the start and was headed only briefly when he came in to make his mandatory pitstop.
After he emerged in the lead following his stop, he pulled out his advantage over Draco's Markus Winkelhock by more than five seconds by the chequered flag.
Porteiro said: "The start was very important. I got some wheelspin and then realised I had to move over on Zuber to protect my position. He was never in front of me."
Winkelhock finished second, despite emerging behind newly crowned World Series champion Robert Kubica after the pitstops. The pair battled for a couple of laps, before Winkelhock got through and pulled away.
Winkelhock said: "Robert just got some massive oversteer on the final corner, which gave me the possibility of passing into Turn One."
Zuber was running comfortably in second but a spin on the outlap after his pitstop meant the race-one winner finished seventh.
Pos Driver Team Time
1. Porteiro Epsilon Euskadi 44:01.856
2. Winkelhock Draco Multiracing Usa + 5.554
3. Kubica Epsilon Euskadi + 6.866
4. Van Lagen KTR + 14.653
5. Montanari Draco Multiracing Usa + 21.206
6. Pavlovic GD Racing + 22.936
7. Zuber Carlin Motorsport + 23.819
8. Pilet GD Racing + 33.384
9. Carbone Eurointernational + 38.830
10. Rigon Victory Engineering + 39.157
11. Pagenaud DHG Saulnier Racing + 39.460
12. Power Carlin Motorsport + 39.953
13. Pasini Cram Competition + 42.588
14. Meneghello Eurointernational + 46.448
15. Miguez Pons Racing + 50.474
16. Fukuda DHG Saulnier Racing + 51.184
17. Tedeschi RC Motorsport +1:01.063
18. Bellarosa Avelon Formula +1:07.170
19. Rees Interwetten +1:22.475
20. Prost Dams + 1 Lap
21. Bennani Avelon Formula + 1 Lap
Driver Team On Lap
Gommendy KTR 18
Maldonado Dams 17
Sharp Jenzer Motorsport 16
Valles Pons Racing 11
Hanley Cram Competition 10
Pellegrino RC Motorsport 5
Fleming Jenzer Motorsport 1
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