Matrin-3-related distal myopathy is characterized mainly by progressive distal weakness of the lower extremities. The mutation p.S85C in matrin-3 (MATR3) has been identified as disease-causing alteration, whereas the specific molecular mechanisms leading to the muscle disease have not been elucidated. In the present study, muscle biopsy samples from six patients and fibroblasts from four patients harboring p.S85C mutation in MATR3 were analyzed. No specific changes in matrin-3 localization or expression were observed. In contrast, localization of the stress granule components G3BP1 and TIA1 was altered and enhanced protein- as well as RNA- expression of G3BP1 and TIA1 was observed in highly dystrophic tissues. Histological changes were pronounced in muscle biopsy specimen taken from distal muscles. In patient-derived primary fibroblasts, cellular response to oxidative stress was monitored using sodium arsenite treatment. The fraction of cells showing stress granule formation upon application of oxidative stress was significantly lower in patients' fibroblasts compared to healthy controls. Similar results were obtained for endoplasmatic reticulum stress using thapsigargin. Stress granule number in stress granule-positive cells with p.S85C mutation was significantly reduced, whereas stress granule size was not markedly altered. Consistently, cell viability upon arsenite treatment appeared significantly reduced in Matrin-3 myopathy derived fibroblasts. In summary, p.S85C mutation in matrin-3 affects the response to cellular stress by impairing stress granule formation and dynamics. This might contribute to cellular damage and progression of dystrophy in muscle of Matrin-3 myopathy.