Primary skeletal muscle ALCL is very rare. Here the authors report a case of skeletal muscle ALCL that was proven pathologically. A 14-year-old boy presented with a persistent fever, chills, night sweats, headache, and significant weight loss. A CT scan of the abdomen showed a hazy mass about 3.2 x 1.2 cm in his left sacrospinalis. Ultrasonography revealed a low-echo and irregular mass in the left lumbar muscle measuring 8 x 1.4 x 3.6 cm in size and a similar mass 8 x 3.5 x 3.7 cm in size in the femoral muscle of the left thigh. MRI demonstrated an abnormal mass signal 4 x 3 x 9 cm in size infiltrating the left sacrospinalis muscle. The biopsy specimen was taken from the femoral muscle of the left thigh at surgery. Histopathological examination revealed a diffuse infiltration of large and atypical cells with pleomorphic nuclei and abundant cytoplasm. Immunohistological staining showed these atypical cells were positive for CD30 (Ki-l), anaplastic lymphoma kinase (ALK), epithelial membrane antigen (EMA), CD3, CD45RO, and CD68. The morphology and immunophenotype were consistent with CD30-positive, ALK-positive, and ALCL of T-cell lineage. The patient's condition was diagnosed as CD30-positive primary skeletal muscle ALCL.