The German, enjoying his first action of the year in Force India’s Mercedes-powered VJM09, lapped in 1m 25.286s on the medium compound tyre to go 0.728s clear of Kevin Magnussen, whose best time came on soft rubber.
Magnussen, making his first appearance behind the wheel for Renault, completed 63 laps in the morning session, though the highest tally once again went to Mercedes' Nico Rosberg, who was third quickest and added a further 74 laps to the 172 he managed on Tuesday.
The German will hand his Silver Arrow over to team mate Lewis Hamilton this afternoon after Mercedes altered their run plans because of the "unprecedented mileage levels" completed in the two days thus far. "We want to keep our boys in top shape for Melbourne!" the team added on Twitter.
Red Bull's Daniel Kvyat and Williams' Felipe Massa ended the morning fifth and sixth on the timesheets as they sampled their respective cars for the first time, just behind Carlos Sainz who caused the first of two red flag stoppages when he stopped his Toro Rosso on the exit of the pit lane with a loose rear wheel.
The other red flag was caused by rookie Rio Haryanto, whose 2016 debut was interrupted when he beached his Manor into the gravel at Turn 5. The Indonesian pilot had clocked 25 laps before that.
Having topped the timesheets on the opening two days of testing with Sebastian Vettel, Ferrari had a difficult morning, with Kimi Raikkonen consigned to the garage for several hours. The Finn eventually emerged with a little over 40 minutes of the session still to run, eventually completing four laps without setting a time.
Wednesday morning's unofficial testing times from Barcelona
|1||Nico Hulkenberg||Force India-Mercedes VJM09||1:25.286||37|
|2||Kevin Magnussen||Renault R.S.16||1:26.014||0.728||63|
|3||Nico Rosberg||Mercedes F1 W07 Hybrid||1:26.084||0.798||74|
|4||Carlos Sainz||Toro Rosso-Ferrari STR11||1:26.239||0.953||66|
|5||Daniil Kvyat||Red Bull Racing-TAG Heuer RB12||1:26.497||1.211||29|
|6||Felipe Massa||Williams-Mercedes FW38||1:26.712||1.426||57|
|7||Felipe Nasr||Sauber-Ferrari C34||1:26.840||1.554||61|
|8||Jenson Button||McLaren-Honda MP4-31||1:26.919||1.633||45|
|9||Romain Grosjean||Haas-Ferrari VF-16||1:26.955||1.669||54|
|10||Rio Haryanto||Manor Racing-Mercedes MRT05||1:29.808||4.522||25|
|11||Kimi Raikkonen||Ferrari SF16-H||No time||4|