The Finn, who finished second in Sunday’s Spanish GP, posted a best time of 1:16.904 on supersoft tyres after 60 laps to lead Ferrari’s Antonio Giovinazzi by 0.068s, who set his best time on hypersofts.
Bottas’ time was over six tenths quicker than the leading time Max Verstappen had set in the Red Bull on day one, but still well down on Lewis Hamilton’s pole lap of 1:16.1 at the weekend’s grand prix.
Giovinazzi’s effort of 1:16.972 was nearly seven tenths better than Sebastian Vettel’s time on Tuesday, although the four-time world champion had set his time on soft tyres, three ‘steps’ harder in Pirelli’s range.
Kevin Magnussen was third in the Haas with a 1:18.274 and nearly two tenths quicker than what team-mate Romain Grosjean managed on the same hypersoft tyres on the first day.
F1 rules dictate that two of the four in-season test days must be given over to drivers with fewer than three grand prix starts and Wednesday’s line-up at the Circuit de Catalunya had plenty of youth on display.
Formula 2 championship leader Lando Norris set McLaren’s best time of the test so far, the 18-year-old Briton producing a lap of 1:18.827 on medium tyres to sit fourth in the timesheet.
That was a tenth ahead of British-Korean Jack Aitken, who completed 63 laps in the morning session on his debut for Renault, and Sauber driver Charles Leclerc.
On his Red Bull debut, 22-year-old Briton Jake Dennis was 10th and completed the second fewest laps of the morning with just 38 on the board.
Fresh from making his grand prix weekend return, Robert Kubica was back in the Williams FW41 but the Pole was 2.3 seconds off Bottas’ leading pace and down in seventh, although four tenths ahead of Nikita Mazepin’s Force India.
McLaren and Force India are the only teams permitted to field two cars, with one apiece specifically being used for Pirelli tyre testing.
Stoffel Vandoorne was carrying out the duty for McLaren on Wednesday and was ninth fastest, while Nicholas Latifi was slowest of all as he carried out the role for Force India.
Sean Gelael racked up the most laps in the morning session as he reached 83 by the lunch break in the Toro Rosso.
The test resumed at 1pm and runs until 5pm UK time.
The final two days of in-season testing takes place at the Hungaroring following the Hungarian GP in late July and immediately before F1’s summer break.
See the final word on all the Spanish GP action and latest F1 talking points on the next edition of the F1 Report – make a date with Sky Sports F1 on Wednesday at 8.30pm Get Sky Sports F1.