To investigate the social roles of countermarking in dogs, we measured tail base position (TBP, a measure of social status), adjacent-marking and overmarking responses of male, nonoestrous female, gonadectomized male and gonadectomized female dogs to controlled presentations of urine from unfamiliar dogs and social groupmates. We also recorded dog sex, TBP, countermarking and urine investigation at a park. In urine presentations, only males overmarked, intact males (but not gonadectomized males) preferentially overmarked intact female urine, overmarking males had higher TBP than males that did not countermark and urine familiarity did not affect overmarking. In contrast, dogs adjacent-marked only unfamiliar samples, and neither sex nor TBP significantly affected adjacent marking. Gonadectomy did not significantly change the likelihood of countermarking. In dog park observations, males and females marked at and investigated ‘scent posts’ comprised of serial countermarks, often associated with visual landmarks. Males and females were equally likely to countermark and investigate urine and countermarks made up a similarly large portion of countermarking for males and females. Males and females with higher TBP urinated, investigated urine, and countermarked more than same-sex dogs with lower TBP. These studies suggest that although intact males may overmark female urine in part to guard mates as previously hypothesized, both sexes, intact and gonadectomized, likely countermark competitively. Social and sexual patterns also suggest that overmarks and adjacent marks may be distinct signals.