Kovalainen, a former McLaren, Renault and Caterham driver and veteran of 109 starts, has not raced since the end of last year.
"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," said the 32-year-old.
Lotus principal Eric Boullier recognised the team had faced a difficult decision, with Italian reserve driver Davide Valsecchi overlooked for the opportunity despite being available.
Praising last year's GP2 winner as a talented youngster with "a great deal of promise", Boullier said they had opted nonetheless for Kovalainen as a seasoned competitor.
"Whilst we have every faith in Davide's abilities, we are obviously involved in a tight Constructors' Championship battle, so it was decided that the experience Heikki could bring to the team would be invaluable as we aim to finish the year in the best position possible," he added.
Lotus are fourth overall but only 26 points behind Ferrari and 37 adrift of Mercedes in second place.
While Kovalainen has not scored a point since 2009, when he was at McLaren, the Finn is a past race winner with four podiums to his credit.
Raikkonen, the 2007 champion who is moving back to Ferrari next season, has complained about back pain this season but has also said Lotus have failed to pay his wages, triggering inevitable speculation about his motives.
(Reporting by Alan Baldwin in London, editing by Clare Fallon)
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