To find morphological characteristics that might identify the sex of Atlantic Puffins (Fratercula arctica), body morphometrics (wing chord, culmen length, bill depth, number of bill grooves, and head-bill length) and presence or absence of a brood patch were compared in a sample of 195 adult Atlantic Puffins of known genetic sex caught on Machias Seal Island, in the Bay of Fundy, Canada. Discriminant Function Analysis (DFA) was performed on 98 birds, and then tested on a separate, independent dataset of 97 birds. The measurements that best predicted sex were culmen length, bill depth, and head-bill length, which correctly classified 81% of our sample. The second-best function included only bill depth and head-bill length, which classified 77% of our sample correctly. Testing the three-predictor discriminant function on a second dataset resulted in a 73% correct classification for the first discriminant equation, and 79% for the second. Classification success was improved to 81% by removing younger birds (with <1.5 grooves), and to ∼90% by further excluding non-breeding birds (those without a broodpatch).