Sousa ended the run of Austrian wild card Sebastian Ofner, who appeared in his maiden semifinal, with a 7-6 (4), 7-6 (4) win. The 62nd-ranked Portuguese will seek his third career title and first since winning in Valencia two years ago.
Earlier, Kohlschreiber held serve throughout against second-seeded Fabio Fognini and defeated the 25th-ranked Italian 7-5, 6-3. Fognini missed out on his second final in as many weeks after winning his fifth career title in Gstaad, Switzerland, last week.
The 47th-ranked Kohlschreiber is 1-2 against Sousa but won their only previous match this season. Both players lost their other final this year, Kohlschreiber to Borna Coric in Marrakech in April and Sousa to Jack Sock in Auckland in January.
Kohlschreiber will appear in his third final at the clay-court event in the Austrian Alps. In 2012 the German was beaten by Robin Haase of the Netherlands in the final, while in 2015 he defeated Paul-Henri Mathieu of France for the title.