Abstract The fossil record of frogs from South America has improved dramatically in recent years. Here we describe a distal fragment of a large-sized humerus recovered from the middle–to–upper Eocene of southernmost Chile. The large distally located ventral condyle, and the presence of two epicondyles (radial and ulnar) confirm its identity as an anuran humerus. Comparisons with humeri from extant and fossil South American neobatrachians suggest a phylogentic affinity to calyptocephalellids (Australobatrachia). If correct, the new fossil represents the first occurrence of this family in high latitudes of South America and the first amphibian recovered from the Magallanes (=Austral) Basin. The humerus also represents evidence for one of the largest frogs known to date from anywhere in the world. Such exceptional body size may reflect an unusually hot and damp palaeoenvironment.