Assortative mating is an important aspect of mate choice, especially in species where both sexes express ornamentation. Such ornaments could function as signals of individual quality and could result in individuals mating with partners of similar quality. We tested for assortative mating by measuring 63 pairs of Atlantic Puffins (Fratercula arctica) at two Canadian colonies (Gull Island, Witless Bay, Newfoundland and Labrador; and Machias Seal Island, New Brunswick), and constructed a function to predict the sex of puffins from Witless Bay. Male and female puffins have similar plumage, and both sexes have fleshy rosettes at the base of their bill, which are supposedly ornaments. We also examined changes in measurements over time in 5–30-year-old puffins recaptured at Machias Seal Island. Our discriminant function correctly predicted the sex of 88 % of puffins from Witless Bay. Overall, males were larger than females in all measurements, but within pairs, some females were larger in 4–27 % of individual measurements. We found no evidence of positive assortative mating or of assortative mating by rosette size, and rosette area did not increase with age. The importance of puffins’ rosettes as indicators of quality requires further investigation.