The team said the 23-year-old, who will partner Britain's Will Stevens in Melbourne, would compete in the opening rounds of the season.
What happens after that remains unclear, with Merhi only recently agreeing a deal to return to the Renault 3.5 series that starts in late April.
Marussia have been more focused on securing their immediate survival than any long-term planning, with the British-based team only recently emerging from administration after missing the last three races of 2014.
Britain's Jordan King, the 21-year-old son of the team's interim chairman and former Sainsbury's chief executive Justin King, was also appointed as a development driver while also competing in GP2.
"It is fantastic to welcome Roberto and Jordan to the team," said principal John Booth of an announcement that comes only three days before first practice at Albert Park.
"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."
Merhi, who raced with Manor in Formula Three Euroseries in 2009 and finished third in last year's Formula Renault 3.5 Series, will be the second Spanish rookie on the grid in Melbourne with Carlos Sainz lining up for Toro Rosso.
Spain's most successful driver, double world champion Fernando Alonso, will be absent however after the McLaren driver crashed in testing.
Merhi took part in three Friday free practice sessions with now-defunct Caterham last season.
The Castellon-born driver said he was excited to be finally stepping up to the starting grid.
"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," he said.
Marussia will be starting the season with a revised 2014 car, tweaked to meet 2015 safety regulations, and last year's Ferrari power unit.
(Editing by Peter Rutherford)
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