The Jenzer Motorsport driver, who lined up 10th for Sunday’s race, took advantage of starting on wets on the damp track as the leaders ran on slicks to make huge moves on the first lap.
He was running second by lap three behind Paul Aron, who started 11th, before the Prema driver pitted for slick tyres.
This handed Barnard the lead, and despite a strong challenge from Christian Mansell (Campos), who had started 23rd, the Briton held on to take his second podium of the weekend.
Nikita Bedrin completed the podium for Jenzer, taking his second rostrum in as many rounds, ahead of team-mate Alex Garcia in fourth to see all three Jenzer cars in the top four.
It was a disappointing race for the title contenders, all of whom started on the slick tyres bar Aron, who finished eighth after the pit gamble failed to pay off.
Poleman Pepe Marti struggled in his Campos-run car, finishing ninth, while championship leader Gabriel Bortoleto finished 11th for Trident ahead of Zak O’Sullivan (Prema) in 12th.
The race, which was shortened from 18 to 15 laps ahead of the weekend, started behind the safety car amid damp conditions after Gabriele Mini (Hitech) crashed on his way to the grid.
Aron immediately made a charge on the wet tyres, firing up the inside of La Source to clear three cars before making it past Trident’s Leonardo Fornaroli and Marti for the lead on the Kemmel Straight.
The frontrunners quickly dropped back, allowing the likes of all three Jenzers, Mansell, Sebastian Montoya (Hitech) and PHM’s Sophia Floersch, who lined up 24th before a fierce charge, to take advantage.
The safety car was deployed on lap four after Trident driver Oliver Goethe had a big smash out of Eau Rouge, with Aron stopping for the slick tyres in an early gamble, handing the lead to Barnard.
Racing resumed on lap eight, with Saturday sprint winner Caio Collet (Van Amersfoort Racing) taking ninth at Turn 1 before passing MP driver Mari Boya for eighth along the Kemmel Straight.
O’Sullivan had been running in 10th, but fell back to 12th after running wide as his slicks struggled for grip.
Mansell and Barnard were embroiled in a fierce fight for the lead on lap 11, with Mansell running wide at Les Combes and going through the run off as they tussled for the lead.
The Campos driver tried another move two laps later, but Barnard put up a staunch defence to hold off his fellow rookie.
Barnard then managed to clear a 1.5s gap to Mansell, with a further six seconds to Bedrin in third.
Though Bortoleto made progress behind, he was unable to get into the points, passing O’Sullivan for 11th on the penultimate lap.
Collet finished fifth ahead of Montoya, with Floersch sixth having made up 18 places.
Aron took eighth after his early pitstop, with Marti in ninth and Franco Colapinto rounding off the top 10 for MP Motorsport.
Bortoleto remains on 144 points heading into the final round, with Aron on 106 in second ahead of Marti on 105.
F3 Spa: Feature race results:
| Cla | Nº | Driver | Team | Gap |
|---|---|---|---|---|
| 1 | 27 | Taylor Barnard | Jenzer Motorsport | |
| 2 | 24 | C.Mansell | Campos Racing | 1.529 |
| 3 | 26 | Nikita Bedrin | Jenzer Motorsport | 7.703 |
| 4 | 28 | Alejandro García | Jenzer Motorsport | 8.915 |
| 5 | 17 | Caio Collet | Van Amersfoort Racing | 20.053 |
| 6 | 14 | Sebastian Montoya | Hitech Pulse-Eight | 20.172 |
| 7 | 29 | Sophia Flörsch | PHM Racing by Charouz | 20.533 |
| 8 | 1 | Paul Aron | Prema Racing | 28.760 |
| 9 | 23 | Pepe Martí | Campos Racing | 30.172 |
| 10 | 10 | Franco Colapinto | MP Motorsport | 31.547 |
| 11 | 5 | G.Bortoleto | Trident | 32.595 |
| 12 | 3 | Zak O'Sullivan | Prema Racing | 33.068 |
| 13 | 11 | Mari Boya | MP Motorsport | 34.910 |
| 14 | 4 | L.Fornaroli | Trident | 35.731 |
| 15 | 7 | Kaylen Frederick | ART Grand Prix | 35.959 |
| 16 | 2 | Dino Beganovic | Prema Racing | 37.752 |
| 17 | 9 | Nikola Tsolov | ART Grand Prix | 40.127 |
| 18 | 19 | Tom Smith | Van Amersfoort Racing | 43.351 |
| 19 | 12 | Jonny Edgar | MP Motorsport | 45.093 |
| 20 | 8 | Gregoire Saucy | ART Grand Prix | 45.875 |
| 21 | 18 | R.Villagómez | Van Amersfoort Racing | 51.388 |
| 22 | 25 | Hugh Barter | Campos Racing | 1'09.043 |
| 23 | 16 | Luke Browning | Hitech Pulse-Eight | 1'38.817 |
| 24 | 21 | F.Simonazzi | Rodin Carlin | 2'17.172 |
| 25 | 31 | Michael Shin | PHM Racing by Charouz | 1 lap /59.109 |
| 20 | Oliver Gray | Rodin Carlin | Retirement | |
| 22 | Ido Cohen | Rodin Carlin | Retirement | |
| 30 | Roberto Faria | PHM Racing by Charouz | Retirement | |
| 6 | Oliver Goethe | Trident | Retirement | |
| 15 | Gabriele Minì | Hitech Pulse-Eight | Not started |
