The 19-year-old German currently drives for the manufacturer's DTM squad, scoring his maiden series win in yesterday's EuroSpeedway Lausitz. He is also a former F3 Euro Series runner-up.
The team confirmed that Wehrlein will "travel with the team from the Singapore Grand Prix onwards as part of his reserve driver duties."
The youngster, who tested for the team at Portiamo,
"For me, it is a nice recognition of my efforts that I have been nominated as reserve driver for Mercedes," said Wehrlein.
"It's a dream come true to have completed this test [at Portiamo] and now to take on my new role.
"All my life, I have worked hard to get the opportunity to drive an F1 car. And it was always my goal to do it with Mercedes-Benz."
Given Wehrlein's increased role at the Mercedes squad and his success in the DTM at a very young age, the German is likely to be linked with an F1 berth in the coming seasons.