American Alexander Rossi has signed a deal to be Marussia's reserve driver for the remainder of the season.
The 22-year-old left rivals Caterham, where he held the same role, last week and begins his role at this weekend's Hungarian Grand Prix.
Rossi said: "Since my initial discussions with the Marussia team management I have felt right at home.
"Marussia is in an exciting place right now and the progress at the team is clear for all to see."
Rossi finished fourth in the GP3 series, a feeder championship to Formula 1, in 2010, and finished third in the Formula Renault 3.5 series a year later.
He progressed to GP2 in 2013, finishing ninth overall, and was 16th in the championship this year for Caterham's GP2 team before leaving for Marussia.
In contrast Marussia, who made their debut in 2010, are enjoying their best season so far and after scoring their first F1 points in Monaco are ninth in the constructors' championship, two places ahead of Caterham.