In 2005, Kovalainen entered the GP2 Series, the new 'feeder' series for F1 and the successor to Formula 3000. Driving for the Arden International team, Kovalainen began the season with strong results by winning the first round of the championship in Imola and then finishing third in the sprint race. In Barcelona he scored his third consecutive podium, but in the sprint race his car stalled on the grid. He dominated the race in Monaco by claiming pole position, leading the race for the first 21 laps and clocking the fastest lap of the race. Problems during his pitstop, however, dropped him down to fifth. At the Nürburgring he gave his best performance yet, by winning the race from 17th on the grid. In the sprint race José María López caused a collision which forced Kovalainen to retire. At Magny-Cours he won again from fourth on the grid, and came third in the sprint race.
He returned to the Race of Champions in 2005, defeating Bernd Schneider and Felipe Massa, but was then knocked out in the semi-finals by Tom Kristensen. Kovalainen won the Nations Cup with Grönholm in 2006, but was eliminated in the individual event at the semi-final stage by Mattias Ekström by a mere 0.0002 seconds. In 2007, Kovalainen and Grönholm again made the Nations Cup final, where they lost to the German team of Michael Schumacher and Sebastian Vettel. Kovalainen beat Vettel in the individual race, before crashing on the finish line in the next round against Andy Priaulx. He crossed the line out of control, and Priaulx pipped him.