It has been rumoured Petrov, the Russian driver ousted by Lotus (formerly Renault) after two seasons, was waiting on his sponsors to decide whether or not to pay out veteran Trulli's contract.
37-year-old Italian Trulli, permanently in F1 since 1997 and the 2004 Monaco grand prix winner, said mere days ago that he expected to race this season.
He tested Caterham's 2012 car at Jerez last week.
"It was not an easy decision to bring Vitaly in to replace Jarno, but it was one we made to ensure that we give fresh impetus across the whole team and with a realistic eye on the global economic market," said team boss Tony Fernandes.
The Malaysian businessman said an "agreement" was reached with Trulli to end his contract, "but he will always be part of our family".
"I understand the decision the team has made and I want to wish to the whole team the very best of luck for the season ahead," said the former Minardi, Prost, Jordan, Renault and Toyota driver.
Petrov, 27, had a seat fitting at Caterham's Hingham factory on Friday and will test the car in Barcelona next week.