The transcriptional repressor B cell lymphoma 6 (BCL6) is highly expressed in skeletal muscle. Although transcriptome-wide studies have shown BCL6 dysregulation in muscular dystrophies, investigations into its endogenous roles in muscle biology remain scarce. We therefore generated skeletal muscle-specific Bcl6 knockout (M-Bcl6 KO) mice and used adeno-associated virus to knockdown (KD) Bcl6 selectively in limb muscles of mice. In both models, Bcl6 deficiency led to reduced muscle mass and contractility. Single-nucleus RNA sequencing and biochemical analyses revealed upregulation of Socs2, and inhibition of the IGF1/AKT pathway. Mitochondrial respiration was significantly reduced in permeabilized myofibers upon Bcl6 KO and KD, and electron microscopy showed decreased mitochondrial density and altered morphology. Pathways regulating mitochondrial quality control were also downregulated. While Bcl6 KO did not significantly impair baseline treadmill running capacity, it blunted the adaptive response to endurance training. These findings demonstrate that Bcl6 is a critical regulator of skeletal muscle mass and mitochondrial bioenergetics, acting through transcriptional control of signaling and metabolic pathways essential for the maintenance of muscle mass and function.