Raikkonen, who had already caused a stir in the morning by trialling the prototype head protection system, swapped onto Pirelli's new purple-marked ultrasoft tyres late on and immediately dropped down to a 1m 22.765s.
That was fractionally faster than team mate Sebastian Vettel's overall best from the first four days, and enough for the Finn to end the session on top by half a second.
Force India's Nico Hulkenberg and Toro Rosso's Max Verstappen also tried the ultrasoft and ended the morning second and third respectively, the duo separated by just over a tenth of a second.
Nico Rosberg was next up for Mercedes, with the world champions once again focusing on longer runs on the medium tyre. Rosberg was the busiest man on track, clocking up over 80 laps.
Sauber's Felipe Nasr finished his final morning in the new C35-Ferrari in fifth, fractions ahead of McLaren's Fernando Alonso, who clocked up 57 laps on another strong morning of running for the British team.
Daniil Kvyat and Felipe Massa were seventh and eighth respectively, as both Red Bull and Williams encountered small issues. The former had to be pushed back to his garage after stopping at the end of the pit lane, while the latter didn't get out in the early part of the day.
Haas, meanwhile, finally had a respite from the gremlins that afflicted them on the first two days of the final test. Romain Grosjean completed close to 60 laps for the team, almost triple Esteban Gutierrez's total from Tuesday and Wednesday - although he also finished the morning beached in the gravel at Turn 4.
The Frenchman wound up 10th overall, just behind Renault's Jolyon Palmer, while Manor's Pascal Wehrlein propped up the timesheets in 11th, a few tenths down on Grosjean.
Thursday's unofficial morning testing times from Barcelona
|1||Kimi Raikkonen||Ferrari SF16-H||1:22.765||41|
|2||Nico Hulkenberg||Force India-Mercedes VJM09||1:23.251||0.486||49|
|3||Max Verstappen||Toro Rosso-Ferrari STR11||1:23.382||0.617||59|
|4||Nico Rosberg||Mercedes F1 W07 Hybrid||1:24.126||1.361||81|
|5||Felipe Nasr||Sauber-Ferrari C35||1:24.760||1.995||37|
|6||Fernando Alonso||McLaren-Honda MP4-31||1:24.870||2.105||57|
|7||Daniil Kvyat||Red Bull Racing-TAG Heuer RB12||1:25.141||2.376||62|
|8||Felipe Massa||Williams-Mercedes FW38||1:25.420||2.655||46|
|9||Jolyon Palmer||Renault R.S.16||1:27.182||4.417||26|
|10||Romain Grosjean||Haas-Ferrari VF-16||1:28.110||5.345||59|
|11||Pascal Wehrlein||Manor Racing-Ferrari MRT05||1:28.476||5.711||19|