Injuries to the likes of Marco Reus, Lukas Nmecha and Timo Werner opened up space for the 17-year-old Borussia Dortmund forward to be included.
Moukoko has scored six goals in 14 appearances for Dortmund this season.
Born in Cameroon, Moukoko has played youth football for Germany but is yet to be capped at senior level for the Europeans.
"I just can't believe it. Last year, around this time I was still out of the window, struggling with many injuries and little playing time," he posted on Instagram.
"Yet, I always tried to stay positive and believe in myself. I knew what I could achieve and now just to be part of a World Cup, a goal that players dream of their entire career, makes me incredibly proud.
"All the hard work has paid off, and I am just incredibly happy and overwhelmed.
"I would like to send a special thank you to my family, Edin Terzic, Sebastian Geppert, Peter Herrmann and the entire Borussia Dortmund coaching team, and my advisor Patrick Williams.
"Thanks also to my teammates and the fans of Borussia Dortmund and everyone who has supported and accompanied me for years. I love you guys."
Germany failed to qualify from their group as defending champions four years ago but will be looking to finish in the first two positions this time in a group that also features Spain, Costa Rica and Japan.
