Congenital Fiber Type Disproportion (CFTD) is primarily characterized by the selective atrophy of slow-twitch (type 1) muscle fibers in the absence of other notable pathological findings. Patients display a broad spectrum of clinical manifestations, ranging from mild to severe, including neonatal hypotonia, delayed motor milestones, spinal and thoracic deformities, as well as life-threatening respiratory muscle weakness. So far, no treatment exists for this form of congenital myopathy. Dominant mutations in the TPM3 gene have been described as a common cause of CFTD, sometimes with nemaline rods; yet attempts to recapitulate the key features of the human disease in transgenic animal models have been unsuccessful. This limitation hinders our comprehension of TPM3-related myopathy, as well as CFTD more broadly, and impedes the exploration of therapeutic interventions. To address this limitation, we utilized CRISPR/Cas9 gene-editing technology to introduce the most prevalent pathogenic mutation (p.R168C) into the Tpm3 gene in mice. Heterozygous mutant mice recapitulated the key features of CFTD in soleus muscles. On average, the diameter of slow muscle fibers was reduced by 50-70% compared to the diameter of fast muscle fibers, mirroring observations in patients. From 2 to 12 months of age, the severity of CFTD tended to worsen primarily driven by the limited growth capacity of slow muscle fibers, and the compensatory hypertrophy of fast muscle fibers. Additionally, heterozygous mutant and homozygous mutant mice showed significant differences in the number of slow muscle fibers per cross-section area. Fiber type disproportion correlated with reduced force production in intact muscles and in single isolated slow muscle fibers. The latter indicates sarcomere-based weakness in slow-twitch muscle fibers, consistent with findings in patients. Transcriptomic analyses are ongoing to dissect molecular events across various muscle groups and fiber types. This study introduces the first CRISPR-based mouse model of TPM3-induced CFTD, faithfully recapitulating key features of the human disease. The model provides a system to study distinct molecular events in slow and fast muscles and serves as a platform to test therapeutic approaches in this ultra-rare disease.