The 34-year-old covered the 4,422-metre run at the Bernese Oberland resort in two minutes, 36.14 seconds to see off Swiss duo and previous Wengen winners Beat Feuz and Carlo Janka.
Feuz finished 12 100ths of a second adrift with Janka, who won Friday's super-combined, 14 100ths away.
Another Swiss Patrick Küng finished fourth.
Kjetil Jansrud, the leader of the overall World Cup standings, took fifth.
Reichelt was adding this to his win in the super-G at Beaver Creek in December to take his career World Cup win tally to nine.
Overall World Cup standings (after 20 events)
1. Marcel Hirscher (AUT) 836 pts
2. Kjetil Jansrud (NOR) 669
3. Fritz Dopfer (GER) 542
Felix Neureuther (GER) 542
5. Alexis Pinturault (FRA) 505
6. Hannes Reichelt (AUT) 432
7. Dominik Paris (ITA) 423
8. Ted Ligety (USA) 420
9. Carlo Janka (SUI) 373
10. Henrik Kristoffersen (NOR) 361
Overall World Cup downhill standings:
1. Kjetil Jansrud (NOR) 339 pts
2. Dominik Paris (ITA) 232
Beat Feuz (SUI) 232
4. Hannes Reichelt (AUT) 225
5. Guillermo Fayed (FRA) 210