Finn Heikki Kovalainen is Lotus's replacement for compatriot Kimi Raikkonen at this weekend's United States Grand Prix and the season-ending race in Brazil.
Discussions involving both Lotus and Caterham finally reached agreement on Wednesday, allowing the former McLaren driver to stand in for the Ferrari-bound Finn.
Securing Caterham's blessing for the deal, because Kovalainen is still under contract until the end of the season, was an important element.
Kovalainen has already visited Lotus' Enstone headquarters to finalise his preparations for a return to racing action for the first time since 2012.
Kovalainen's deal came after efforts to recruit Germans Nico Hulkenberg of Sauber and former champion Michael Schumacher, now in retirement, came to nothing.
Schumacher's manager Sabine Kehm told German newspaper Bild: "There have been quite a few requests in the past. Michael would still be fit enough, but he has a new life. I don't find it unusual that people ask.
"Many people changed their assessment about Michael because of Nico Rosberg's recent performances compared to those of Lewis Hamilton.
"Michael drove as well as Nico, just as Nico is driving as well as Lewis."
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