A tensioning apparatus or device for a flexible driving means, particularly a drive chain for an internal combustion engine, comprises a housing, a tensioning piston movably guided in said housing and an adjusting or regulating means for gradually adjusting the working stroke of the tensioning piston. The adjusting means comprises a locking bracket and an appertaining locking profile at the tensioning piston, two arch-shaped clamping sections of said locking bracket being engageable with said locking profile, and a resilient spring portion with a torsional area combining the clamping sections in spring-resilient manner. In a preferred embodiment, the locking bracket comprises a U-shaped section with a portion angled towards the housing wall and forming the torsional area. The housing is further provided with a guide projection projecting from said housing wall for axially guiding said angled portion of said U-shaped section of said locking bracket in the working stroke of said tensioning piston in torsion-proof manner.