Kamui Kobayashi (JPN) Caterham in the Press Conference. Formula One World Championship, Rd11, Hungarian Grand Prix, Preparations, Hungaroring, Hungary. Thursday, 24 July 2014
Merhi will take Kobayashi's place for FP1, which will be the 23-year-old's first run in F1 machinery.
"I am delighted to be taking part in my first ever Formula One free practice session and I want to thank Caterham F1 Team for this opportunity - it's something I've wanted for a very long time," Merhi said.
"I've been working very closely with the team during the last few weeks and I feel ready for this opportunity; Friday is going to be a very special day for me and one I will never forget."
Merhi makes the step up to F1 racing having taken two victories so far - and a podium in the Monza season opener - in this year's Formula Renault 3.5 championship. He previously stormed to the 2011 European Formula Three title, before spending two seasons driving for Mercedes-backed teams in Germany's DTM touring car series.
"What a year it's been so far," he added. "Last Christmas I didn't really know if I was going to carry on racing and now I'm about to drive a Formula One car! I'm definitely going to make the most out of this weekend in Italy."
Kobayashi, meanwhile, returns for the Formula 1 Gran Premio d'Italia 2014 after sitting out the previous round at Spa-Francorchamps to make way for three-time Le Mans winner Andre Lotterer.
"I am working for Caterham F1 Team and I always respect team decisions," Kobayashi said. "Now I am back in the car and looking forward to going racing again!
"We have worked hard as a team and added performance to the car as well as bringing new updates since I last drove in Hungary before the summer break, so I'm looking forward to feeling the updates on track here in Monza."