It was shortly after the break that Ridie Baku nicely found Lukas Nmecha with a pass towards the goal.
Nmecha, who is on loan at Belgian Anderlecht from Manchester City, rounded goalkeeper Diogo Costa and put the ball in.
This is the third time that Germany takes gold in the U21 European Championships.
The most recent was 2017 in Poland and before that in Sweden in 2009.
- It was not what I had predicted, but as so many times before, Germany is there as the final winner, says Johannesson.
Portugal has lost finals before and most recently it was on penalties against Sweden in 2015.