Background: Instability at the ulnocarpal joint has many causes, but the common thread among these causes is the presence of abnormalities in the triangular fibrocartilage complex (TFCC). However, the biomechanical consequences at the ulnocarpal joint after detachment of the TFCC from the ulnar styloid are not clearly defined. Better delineation of whether peripheral TFCC detachments cause ulnocarpal instability will help to design surgical treatments. Questions/Purposes: We asked whether detachment of the peripheral TFCC from the ulnar styloid causes ulnocarpal instability. Methods: Using 20 fresh-frozen below-elbow cadaver specimens, the distal ulna was cycled volarly and dorsally with the carpus held firmly. The load-displacement curve was analyzed to determine the resistance of the ulnocarpal joint against dorsal-volar displacement of the ulna (stiffness) and the amount of dorsal-volar excursion with minimal resistance before reaching firm end points dorsally and volarly. A standardized 3-mm transection of the attachment of the TFCC from the ulnar styloid was created with a scalpel using arthroscopic observation. Mechanical testing was repeated and paired Student's t-tests conducted. Results: The mean stiffness of the ulnocarpal joint was decreased after detachment. The amount of dorsal-volar excursion was similar after detachment of the peripheral TFCC. Conclusions: There is decreased stiffness at the ulnocarpal joint after detachment of the peripheral TFCC, but there is no biomechanically detectable difference in dorsal-volar excursion. Clinical Relevance: The findings of the current study can be used to develop and evaluate innovative surgical techniques, such as capsulorraphy or ligamentous reconstruction, that specifically address laxity at the ulnocarpal joint after peripheral TFCC detachment.