The German driver has already worked for the Mercedes F1 team this season, completing more than 30 days in their simulator, and he drove a Formula One car for the first time last week at Portimao as part of a young driver evaluation test, completing 500 kilometres in a 2012 Mercedes F1 W03.
“We are delighted to officially welcome Pascal to the team after his successful test last week in Portugal,” said Toto Wolff, head of Mercedes-Benz motorsport.
“He has worked hard behind the scenes this year in our simulator, playing a very important role in our pre-race preparations. Aside from Nico and Lewis, he is the driver most familiar with all the procedures of our F1 W05 Hybrid and therefore the right choice for the role of reserve driver.
“He took an important step last week when he drove a Formula One car for the first time - and we were pleased to see him deliver a composed and impressive test, in line with our expectations. This was followed by a fantastic win yesterday at the Lausitzring in the DTM, making the most of conditions where pure driving talent was able to shine.
“Pascal has a bright future ahead of him and we are excited to have him onboard for what will be an intense conclusion to the Formula One season."
Wehrlein described his new role as a “dream come true,” adding: “I was delighted when I learned that I would be completing a Formula One test for Mercedes in Portimao - [it] was simply the greatest experience I have had so far in motorsport. There weren't any nerves, which surprised me - I just felt ready for it.
“The feedback from the team and from Toto was very encouraging and I think that I did a good job. It certainly helped that I have been working hard in the simulator since February.
“For me, it's a dream come true to have completed this test 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.”
Wehrlein joined the Mercedes stable after success in a variety of single-seater championships - he took the 2011 ADAC Formel Masters title and then graduated to Formula Three, winning once, finishing second overall and claiming top rookie honours in his first year in the F3 Euroseries. He moved into DTM in 2013.
Wehrlein will travel with Mercedes to the final six Grands Prix on the 2014 F1 calendar.