Merhi has previous experience in Formula One racing, having driven for Caterham in three FP1 sessions during the 2014 season. Still only 23-years-old, he has enjoyed success in a number of disciplines including winning the 2011 European Formula Three crown and finishing third in last year's Formula Renault 3.5 championship.
"I am incredibly excited to be making my Formula One debut in Melbourne this weekend," Merhi said. "It is a big responsibility but I feel ready to make this step in my career and to show that I can make an important contribution to developing a team."
Alongside Merhi's appointment, Marussia also announced 21-year-old Briton Jordan King as their new development driver.
"It is fantastic to welcome Roberto and Jordan to the team," Marussia team principal John Booth said. "Not only are they very talented young drivers with exciting futures ahead of them, they have also been 'graduates' of Manor Motorsport in the junior formulae, which is obviously very rewarding for us, given that the development of young talent is an important part of our philosophy."
King, who will combine his new duties with a full season competing in GP2, added: "I am obviously over the moon to be joining Marussia as a development driver. This is such an exciting project, full of great people and with a real chance of success. This is another great step closer to my ultimate goal of racing in Formula One."
Marussia announced Will Stevens as their first 2015 driver in February.