Simple SummarySoft tissue sarcomas (STSs) comprise different tumor subtypes with similar clinical and pathological characteristics that arise from the embryonic mesoderm. They affect both humans and dogs and are located in the cutaneous or subcutaneous tissue of different body structures. In humans, this tumor group has been well-studied and described, and several clinical and pathological factors are used to evaluate patient prognosis, including a clinicopathological unified staging system. However, in dogs, no staging system has been proposed for these tumors. Therefore, we adapted human clinicopathological staging for STS and applied it to a large group of dogs affected by STS. The staging system proposed ranged from stage I to IV and was able to separate patient prognosis, with stage I patients having a higher survival time and stage IV patients having the lowest survival time. Moreover, we investigated different clinical and pathological factors associated with prognosis in this set of patients. We then evaluated the median value, which was five for the mitotic count. Therefore, we evaluated whether this value could separate patient survival. It was possible to identify a higher survival time in patients with a mitotic count ≤5 than in those with a count >5.AbstractSoft tissue sarcomas (STSs) are a heterogeneous group of malignant mesenchymal tumors with similar histological features and biological behaviors. They are characterized by a low to moderate local recurrence rate and low metastasis, affecting approximately 20% of patients. Although this tumor set is vital in veterinary medicine, no previous unified staging system or mitotic count has been associated with patient prognosis. Therefore, this study proposed a new clinicopathological staging method and evaluated a cut-off value for mitosis related to the survival of dogs affected by STS. This study included 105 dogs affected by STS, treated only with surgery, and a complete follow-up evaluation. The new clinicopathological staging system evaluated tumor size (T), nodal involvement (N), distant metastasis (M), and histological grading criteria (G) to categorize the tumor stage into four groups (stages I, II, III, and IV). The proposed tumor staging system was able to differentiate patients’ prognoses, with dogs with stage IV disease experiencing the lowest survival time and dogs with stage I disease having the highest survival time (p< 0.001). Moreover, we assessed the median mitosis (based on mitotic count) and its association with overall survival. Our study’s median mitosis was 5, and patients with ≤5 mitoses had a higher survival time (p= 0.006). Overall, the proposed staging system and mitotic count seemed promising in the prediction of patient prognosis.