The Pole completed 114 laps of the Spanish track, which was initially wet after heavy rain in the morning.
"I was able to get to grips with the DTM Mercedes AMG C-coupe and felt very comfortable in the car, even though after my long break it meant adjusting to the high downforce that a state-of-the art DTM race car produces," he said to Autosport.
"I'm very pleased with my performance on this first day of DTM testing and had no problems whatsoever in the car."
Kubica has yet to return to circuit racing following his horrific rallying accident in Italy almost two years ago.
The Pole told F1 Racing magazine that he feels he will one day be able to drive an F1 car. Kubica raced in F1 between 2006 and 2010, winning the 2008 Canadian Grand Prix for BMW Sauber.