Adult Rough-legged Hawks (Buteo lagopus) are depicted and described in bird guides and handbooks as sexually dimorphic in plumage, although several authorities have reported that the sexes overlap in plumage. We examined photographs of 106 adult and Basic III Rough-legged Hawks that had been captured and measured, and more than 200 museum specimens to assess sexual dimorphism in five plumage characters and in wing chords. On the basis of museum specimens with gonad information on their labels, the sexes are distinguishable by wing chord, which is >420 mm in females and <418 in males. Most light-morph adults have the plumage of their sex as described in field guides and handbooks, but a few appear like the other sex. Gray barring on the back feathers and barring on the flanks (present in males, absent in females) are the best plumage indicators of the sex of birds in Definitive and Basic III plumages. Tail pattern was the most shared character and is not a reliable indicator of adults' sex.