"Thanks Caterham for the opportunity," the Belgian-raised German wrote on Instagram as the news broke officially. "Thanks Audi Team TOM's for the freedom."
Although a F1 rookie at the age of 32, Lotterer - with links to new team advisor Colin Kolles - is a multiple Le Mans winner, F3 and Formula Nippon champion and former Jaguar F1 tester.
"The team will benefit from having another experienced driver at the wheel of the car throughout the weekend," the team said in a media statement.
"The weather also plays a big role at Spa-Francorchamps and Lotterer is one of the most experienced drivers racing under mixed conditions," Caterham added.
As for Kobayashi's future, Caterham said the popular 27-year-old is sitting out Spa but the "Japanese driver still remains part of the team".