Duchenne muscular dystrophy (DMD) is characterized by progressive skeletal muscle degeneration. No treatments are currently available to prevent the disease. While the muscle enriched microRNA, miR-133b, has been implicated in muscle biogenesis, its role in DMD remains unknown. To assess miR-133b function in DMD-affected skeletal muscles, we genetically ablated miR-133b in the mdx mouse model of DMD. In the absence of miR-133b, the tibialis anterior muscle of juvenile and adult mdx mice is populated by small muscle fibers and exhibits increased fibrosis, characterized by thickened interstitial space. Additional analysis revealed that loss of miR-133b exacerbates DMD-pathogenesis partly by altering satellite cell numbers and through widespread transcriptomic changes. These include known miR-133b targets as well as genes involved in cell proliferation and fibrosis. Altogether, our data demonstrate that skeletal muscles utilize miR-133b to mitigate the deleterious effects of DMD. Overall design: Number of replicates for each sample: Control, 4; miR-133b-/-, 4; mdx, 4; mdx/miR-133b-/-, 4