"I had a brake failure so there was really no point in trying to continue," he said of his first retirement since he returned to Formula One last season after two years out.
"We've finished a lot of races and had some good reliability; one day your luck has to run out and today was that day."
The Finn tried to overtake Ferrari's Brazilian Felipe Massa in a battle for seventh place on lap 26 when he ran off the track while braking.
The 2007 world champion, who passed Michael Schumacher's 2001-03 record of 24 successive races in the points earlier in the season, returned to the pits and got out of the car.
Team principal Eric Boullier said only the left brake was affected, with the cooling duct blocked. The team are investigating.
Raikkonen, a four-times winner in Belgium, was second in the championship before Sunday's race won by Red Bull's triple champion Sebastian Vettel but dropped to fourth and 63 points adrift.
The retirement also ended his hopes of beating Nick Heidfeld's record run of 41 successive race finishes. The Finn had 38 before Sunday.