Kovalainen was dropped as a Caterham race driver at the end of last season as the Malaysian-owned team opted for an all-new and inexperienced line-up of Frenchman Charles Pic and Dutch rookie Giedo van der Garde.
Caterham, who have yet to score a point in more than three years, have fallen behind rivals Marussia in performance in the three races so far.
"It's obviously great news that I'll be on track in Bahrain and Spain and I'm really looking forward to getting back to work and helping the team as much as I can," Kovalainen said in a statement.
The Finn has six years of F1 experience and is a former race winner whose technical insights will be invaluable for the struggling team.
"I'm physically ready to get back into the car and I know my experience can help give the team new ideas and options for directions to take on setup and strategy," he said.
Team principal Cyril Abiteboul added: "He is perfectly placed to provide us objective feedback on the various types of configurations we will run and to give us an objective view of the 2013 tyres against his experience with the 2012 compounds.
"Having invested in Heikki for our first three years of competition it would be a waste not to leverage the valuable expertise he brings."
American Alexander Rossi remains the team's other reserve and will compete in the junior GP2 series.
The team said "new opportunities" had arisen for China's Ma Qing Hua, who took part in Friday practice in his home grand prix last weekend.
"He remains a core member of Caterham F1 Team's driver roster and an announcement about his revised race plans in 2013 will be made in due course," the team added.
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