Theaux navigated the fast course at Garmisch in 2 minutes, 1.38 seconds, 7/100ths of a second ahead of Swiss veteran Didier Cuche, with Aksel Lund Svindel of Norway right behind.
The run was the final practice session before the championship race on Saturday.
Canadian Erik Guay had the fourth-fastest run, followed by Austria's Michael Walchhofer, who is among the favorites in Saturday's field.
Travis Gong was the top U.S. finisher with a 2:02.20. Bode Miller was clocked at 2:03.90.