It was the Spaniard's third pole of the season. He lapped the Italian Grand Prix venue in 1:36.359, leading the timesheets of the first group of runners.
Adrian Valles, fighting for second in the Championship, will line up alongside Porteiro after being the fastest of his group of runners.
The average of the fastest times for the session was outside the permitted limit for the grid to be decided on lap times alone, so as per series rules both groups of runners will start parallel to each other.
Colin Fleming will line up third ahead of Patrick Pilet, who recorded his best qualifying result of the season.
Eric Salignon will have to start 17th despite qualifying seventh, because of the grid penalty he suffered after his stewards judged he was the guilty party in a first lap accident at Donington Park last month.
Christian Montanari, Jaap Van Lagen and Championship returnee Giorgio Mondini were among several drivers whose session ended early after accidents.
Pos Driver Time 1. Felix Porteiro 1:36.359 2. Adrian Valles 1:36.762 3. Colin Fleming 1:36.531 4. Patrick Pilet 1:36.908 5. Tristan Gommendy 1:36.802 6. Giorgio Mondini 1:37.210 7. Eric Salignon 1:37.009 * 8. Ryan Sharp 1:37.608 9. Markus Winkelhock 1:37.017 10. Celso Miguez 1:37.684 11. Milos Pavlovic 1:37.302 12. Robert Kubica 1:37.759 13. Davide Rigon 1:37.642 14. Damien Pasini 1:37.778 15. Matteo Meneghello 1:37.654 16. Jaap Van Lagen 1:37.877 17. Danny Watts 1:37.775 18. Ferdinando Monfardini 1:37.984 19. Ryo Fukuda 1:37.890 20. Andreas Zuber 1:38.376 21. Simon Pagenaud 1:37.911 22. Giovanni Tedeschi 1:38.683 23. Pastor Maldonado 1:38.016 24. Fernando Rees 1:38.965 25. Matteo Pellegrino 1:38.355 26. Carlos Iaconelli 1:39.469 27. Richard Antinucci 1:40.170 28. Christian Montanari 1:39.785 * Will incur penalty of 10 places
The 2015 Formula Renault 3.5 season will be notable for being the last with the French company's backing but also produced a worthy champion in Oliver Rowland. PETER MILLS evaluates this year's crop of drivers
Last year's field was extraordinary and hard to match, but the Formula Renault 3.5 grid will still have plenty of talent this season. Glenn Freeman runs through the field ahead of this weekend's season opener
Landing the Fortec seat that took rising star Robin Frijns to the 2012 FR3.5 title was already a good sign for Stoffel Vandoorne. Then McLaren took him under its wing. Peter Mills tells Vandoorne's story so far