The 23-year-old will line up alongside England's Will Stevens on the grid for this weekend's Australian Grand Prix in Melbourne.
The team was only cleared to race last week with a modified version of a 2014 car after coming out of administration.
"I hope to pay back the hard work of the team with a solid weekend," Merhi said.
The Spaniard, who raced in the Formula Renault 3.5 Series last year with Zeta Corse, also gained some F1 experience in 2014 with three free practice outings for Caterham.
Merhi will compete in the "opening rounds" of the season, according to the former Marussia team, who have secured investment from Stephen Fitzpatrick, boss of energy firm Ovo, and have former Sainsbury's boss Justin King as interim chairman.
King's 21-year-old son Jordan will also join the team as a test driver, combining his duties with competing in GP2 - F1's feeder series.
"This is another great step closer to my ultimate goal of racing in Formula 1," Jordan King said.
Manor Marussia have named Spain's Roberto Merhi as their second driver for the start of the 2015 season.