For Debian 9 and newer, the Nitrokey App is included in the Debian repository already. To install execute "sudo apt install nitrokey-app".
If you enable the backports repository you can install more recent versions. See here for instructions to enable backports.
More recent versions
As the Nitrokey App version in the official Debian repositories are quite old, one can add the Nitrokey repo built for Ubuntu by typing the following in a terminal (tested on Debian Stretch):
echo 'deb http://ppa.launchpad.net/nitrokey-team/ppa/ubuntu xenial main' > nitrokey-team.list
sudo mv nitrokey-team.list /etc/apt/sources.list.d/
sudo apt-get update
sudo apt-key adv --recv-key --keyserver keyserver.ubuntu.com E72D7141
sudo apt-get install nitrokey-app