Hamilton, who has a 24-point lead over Sebastian Vettel as the Formula One season roars back into life at the historic Spa-Francorchamps Circuit following its traditional one-month summer shutdown, finished 0.168 seconds slower than Raikkonen.
Valtteri Bottas, due to start towards the back of the field tomorrow following a series of grid penalties for exceeding engine parts, was third for Mercedes ahead of the Red Bull of Max Verstappen.
Vettel finished at the top of the time sheets in the opening running, but settled for only fifth in the afternoon, three quarters of a second off his Ferrari team-mate’s pace.
Hamilton is armed with a new engine this weekend at the high-power 4.35-mile track, but Ferrari’s pace in both sessions suggests that the Italian team may hold the advantage heading into the remainder of the weekend. Indeed, Ferrari traditionally hold a little in reserve during practice.
Red Bull’s Daniel Ricciardo spent all but the final minutes of the first session in the garage with an engine issue.
The Australian, who stunned Red Bull by announcing he will leave the team to join Renault next year, had a trouble-free second session and finished sixth, nine tenths down on Raikkonen.
Sergio Perez finished seventh for the re-branded Racing Point Force India team ahead of Carlos Sainz in the Renault as the two Sauber cars of Marcus Ericsson and Charles Leclerc completed the top 10.
Earlier, British teenager Lando Norris made his grand prix weekend debut for McLaren as he deputised for Fernando Alonso in the opening 90-minute session.
The 18-year-old, who is in the frame for a full-time drive with the struggling British team next year, finished 18th of the 20 drivers, three seconds slower than the front-runners.
Norris, however, was faster than Stoffel Vandoorne in the sister McLaren after the Belgian finished last. Vandoorne, whose future at McLaren is under increasing scrutiny, was also bottom of the pile in the second running.
Alonso, who will bring the curtain down on his 17-season career at the end of the year, finished 16th.