Populations of Leach's Storm-Petrels (Oceanodroma leucorhoa) are in decline globally, but causes of these declines are unclear. One possibility is predation at breeding colonies. To estimate effects of one pair of Great Horned Owls (Bubo virginianus) on a Leach's Storm-Petrel colony, we analyzed owl pellets collected from Bon Portage Island, Nova Scotia, Canada, over 5 consecutive breeding seasons (2011–2015). Great Horned Owl diets were mainly Leach's Storm-Petrels by number (79%) and by mass (66%). Muskrats (Ondatra zibethicus), meadow voles (Microtus pennsylvanicus), and masked shrews (Sorex cinereus) completed the diet. This is, to our knowledge, one of the most specialized diets reported for this species of owl. We estimate that this specialized diet reduced by at most 1% annually the population of Leach's Storm-Petrels on this island.