Objective: To understand DNAJB6's function in skeletal muscle and identify therapeutic targets for limb-girdle muscular dystrophy 1D (LGMD1D). Methods: DNAJB6 knockout (KO) myoblasts were generated with Crispr/cas9 technology, and differentially accumulated proteins were identified using stable isotope labeling, followed by quantitative mass spectrometry. Cultured KO myotubes and mouse muscle from DNAJB6b-WT or DNAJB6b-F93L mice were analyzed using histochemistry, immunohistochemistry, and immunoblot. Mouse functional strength measures included forelimb grip strength and inverted wire hang. Results: DNAJB6 inactivation leads to the accumulation of sarcomeric proteins and hypertrophic myotubes with an enhanced fusion index. The increased fusion in DNAJB6 KO myotubes correlates with diminished glycogen synthase kinase-β (GSK3β) activity. In contrast, LGMD1D mutations in DNAJB6 enhance GSK3β activation and suppress β-catenin and NFAT3c signaling. GSK3β inhibition with lithium chloride improves muscle size and strength in an LGMD1D preclinical mouse model. Conclusions: Our results suggest that DNAJB6 facilitates protein quality control and negatively regulates myogenic signaling. In addition, LGMD1D-associated DNAJB6 mutations inhibit myogenic signaling through augmented GSK3β activity. GSK3β inhibition with lithium chloride may be a therapeutic option in LGMD1D.