Inclusion body myositis (IBM) is an acquired myopathy of both inflammatory and degenerative nature. Here we report a case of IBM associated with prolonged use of imatinib not reported in the literature so far. An 81 years old male with a history of gastrointestinal stromal tumor (GIST) operated 8 years ago was evaluated for the progressive loss of weight and muscle strength leading to total immobilization in 6 months. He was under imatinib for 8 years despite the remission of GIST. Physical examination disclosed diffuse loss of muscle strength, most prominently involvement of distal upper and proximal lower extremity in an asymmetrical pattern with normal serum creatinine kinase level (CK). Further investigations including bilateral thigh MRI, electromyography (EMG), and PET/CT suggested myositis and degenerative myopathy and ruled out any malignancy. Quadriceps femoris biopsy proved the diagnosis of IBM and no trigger except for imatinib was displayed. Clinical improvement in terms of wieght loss and muscle weakness was achieved after discontinuation of imatinib. Since imatinib is widely used in different conditions, it is important to be aware of even its rare adverse effects. Poor response of IBM to conventional immunosuppressive agents enhances the value of etiology identification to relieve symptoms in addition to supportive care.