The Spaniard put in a late surge to post a best time of 2:11.434s around the cold and slightly damp 7.004km current Formula One circuit. Italian driver Mirko Bortolotti was second fastest for the session with Brit Jack Clarke, who secured a best result of fifth when he raced here in 2009, third fastest.
Will Bratt, who earlier this month headed the timesheets during the test session at the all-new Snetterton 300 circuit, was fourth with Mihai Marinescu rounding out the top five.
Dutchman Kelvin Snoeks produced one of his best performances so far this season to go sixth quickest with Tobias Hegewald, who dominated the 2009 F2 races here, in seventh and championship leader Christopher Zanella in eighth. Thiemo Storz, another strong runner at the Snetterton 300 test, and Miki Monras completed the top 10.
Pineiro, who set the fastest lap of the race when he raced here during the 2008 Formula BMW Europe series, said: "The track was very tricky, there were three corners that were still wet, but I got a good set up and it's the first time I've raced here with a low downforce set up.
“My tires were good in the session but the track was still damp. I wasn't pushing my hardest because I knew the difference on the damp area would be the key point, but hopefully it will be dry in the next session and we'll keep working."
Free Practice Two gets underway at 14:05 (local time).
Spa results, Free Practice One, Top-10 only:
1. Ramon Pineiro, 2:11.434s
2. Mirko Bortolotti, +0.226s
3. Jack Clarke, +0.469s
4. Will Bratt, +0.565s
5. Mihai Marinescu, +0.732
6. Kelvin Snoeks, +1.035s
7. Tobias Hegewald, +1.069s
8. Christopher Zanella, +1.181s
9. Thiemo Storz, +1.752s
10. Miki Monras, +1.853s