Inclusion body myositis (IBM) is a late-onset myopathy that manifests as distinct muscle weakness in the quadriceps, finger flexors, and ankle dorsiflexors. T-cell large granular lymphocyte (T-LGL) leukemia is a late-onset clonal disorder of CD8+ cytotoxic T-cells that is often accompanied by autoimmune diseases. To date, the association between IBM and T-LGL leukemia has been infrequently reported. Here, we report a case of a patient with T-LGL leukemia who developed IBM, along with in-depth laboratory, electrophysiological, and pathologic findings.