Raikkonen announced on Sunday he would be pulling out of the races in the United States and Brazil to undergo back surgery to cure a long-standing problem.
After failing to lure Michael Schumacher out of retirement for a second time, and Nico Hulkenberg from Sauber, Lotus have turned to Kovalainen who has competed in 109 grands prix, with his last a year ago for Caterham.
Team principal Eric Boullier has had to act fast in recruiting the 32-year-old who has previously raced for Renault and McLaren, along with Caterham where he has been reserve this year, taking part in six Friday practice sessions.
After completing a seat fitting at the team's factory in Enstone on Wednesday, Kovalainen then flew directly to Austin, the scene of this weekend's United States Grand Prix.
"It is a fantastic opportunity for me to join Lotus for the final two races of 2013," said Kovalainen.
"We've seen this year the car can win races and finish on the podium, so I will be pushing hard for the best results possible.
"Jumping into a car so late in the year, when you have not been competing in the races all season, will be a challenge.
"But I know the team at Enstone well so I have no concerns about getting up to speed.
"This is a great opportunity for me, so I would like to thank Tony Fernandes (Caterham owner) and Caterham F1 Team for allowing me to take advantage of it.''
With Lotus still battling Mercedes and Ferrari for second in the constructors' championship, and with millions of pounds at stake, Boullier felt Kovalainen's experience would better serve Lotus compared to promoting reserve Davide Valsecchi.
"Obviously we had to move quickly following the news of Kimi's non-participation in the final two races of this season," said Boullier.
"We found ourselves facing a difficult decision in terms of who should take the wheel in Austin and Sao Paulo.
"On the one hand we had our reserve driver Davide Valsecchi - who is a talented young driver that has shown a great deal of promise - and on the other we had the opportunity to bring in a seasoned Formula One competitor in Heikki.
"Whilst we have every faith in Davide's abilities, we are obviously involved in a tight constructors' championship battle.
"So it was decided the experience Heikki could bring to the team would be invaluable as we aim to finish the year in the best position possible.
"We must thank Tony Fernandes and Caterham for their professional conduct in allowing Heikki to join us for the final races of this season.''