Mercedes announced the appointment a day after the teenager won from pole position for Mercedes in a DTM race at the Lausitzring and only four days after his first test in a grand prix car.
The DTM championship has now been decided, with Germany’s Marco Wittmann winning for BMW with two races to spare.
Wehrlein’s new F1 role means he will attend this weekend’s Singapore Grand Prix and future races as a potential replacement should championship leader Nico Rosberg or Britain’s 2008 champion Lewis Hamilton be unable to race.
Wehrlein has already worked with the team this season, completing more than 30 days in the Formula One simulator and driving more than 12,000 km at the wheel of the virtual F1 W05 Hybrid car.
He drove a Formula One car for the first time at Portugal’s Algarve circuit last Thursday when he tested the 2012 Mercedes and completed 500km.
“He has worked hard behind the scenes this year in our simulator, playing a very important role in our pre-race preparations,” said Mercedes motorsport head Toto Wolff.
“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.
“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,” added the Austrian.
Rosberg and Hamilton are in a duel of their own for the championship, with the German 22 points clear of the Briton with six races remaining.
(Reporting by Alan Baldwin, editing by Ossian Shine)