Recent taxonomic revisions of the Wandering Albatross sensu lato has resulted in four separate species, the rarest of which is the Tristan Albatross (Diomedea dabbenena). We present the first detailed morphometric measurements of the male and female of the Tristan Albatross. The results are used to separate this species from the more common nominate taxon of Wandering Albatross (Diomedea exulans) from South Georgia. The Tristan Albatross is smaller than the Wandering Albatross in all measured variables, and males are larger than females in both species. A combination of bill measurements discriminated 97-98% of individuals of the two species, and tarsus and bill measurements allowed the sex of birds from both species to be determined. If the sex of the bird is known, then bill length should identify all individuals to species. This result is useful to allow fishery observers to correctly identify albatrosses killed on longlines in the Atlantic Ocean. However, care has to be taken outside this region, because the Tristan Albatross is very similar in size to published measurements of Gibson’s Albatross (Diomedea gibsoni) and the Antipodes Albatross (Diomedea antipodensis), both of which breed off New Zealand and are thought to range across the South Pacific Ocean.