Adequacy of surgical resection margins for soft tissue sarcomas are poorly defined because of the various classifications and definitions used in prior studies of heterogeneous patient cohorts and inconsistent margin sampling protocols. Surgical resection margins of 166 primary, high-grade, pleomorphic sarcomas of the extremity or trunk were classified according to American Joint Committee on Cancer R and Musculoskeletal Tumor Society categories, as well as by metric distance and tissue composition. None of the cases were treated with neoadjuvant therapy. Multivariable competing risk regression models were evaluated and optimal surgical resection margins for each classification system were defined. Minimum safe tumor clearance was 5 mm without use of adjuvant radiotherapy and 1 mm with adjuvant radiotherapy. Predictive accuracy of margin classification systems was compared by area under receiver-operating characteristic curves generated from logistic regression of 2½-year local recurrence-free survival and other standard tests of diagnostic accuracy. The Musculoskeletal Tumor Society and margin distance classifications performed similarly, both of which showed higher sensitivity and negative predictive value compared to the American Joint Committee on Cancer R classification. The prognostic power of close or positive margins in prediction models significantly increased when six or more slides were submitted for assessment of surgical resection margins. Surgical resection margins for soft tissue sarcoma should be reported using the Musculoskeletal Tumor Society classification or metric distance to the closest resection margin. Musculoskeletal Tumor Society wide/radical margins or tumor clearances of 5 mm (without adjuvant radiotherapy) or 1 mm (with adjuvant radiotherapy) appear to define the minimum safe surgical resection margins necessary to decrease the likelihood of local recurrence of high-grade pleomorphic sarcomas of the extremity or trunk.