Nonbreeding season territoriality is found in many mammal species irrespective of diet, being found in carnivores, granivores, and herbivores. However, we know the hormonal basis for this behavior in virtually none. American martens (Martes americana) show strong intrasexual, year-round territoriality. We collected serum samples from both territorial and transient males and documented size metrics and movement patterns with live-capture and radiotelemetry from September to June in 3 years in Southeast Alaska. Dehydroepiandrosterone (DHEA—a prohormone that can be converted in the brain to active gonadal hormones) levels were high (comparable to humans and rodents showing nonbreeding season spacing behavior) and did not change over the nonbreeding season in territorial males. In contrast, testosterone levels were low in autumn and increased as the summer breeding season approached. Territorial males were larger than transients in some measures and had higher testosterone levels, but similar DHEA and cortisol levels (total and free). DHEA levels declined with age. Our evidence is consistent with the hypothesis that DHEA, not testosterone, is the basis for territoriality outside the breeding season in martens, but collection of serum from females and experimental manipulations are required to provide conclusive evidence.