Monteiro's Storm-petrel (Hydrobates monteiroi) is a small seabird endemic to the Azores archipelago, where it is known to breed only on two mammal-free islets. Total breeding numbers are between 250 and 300 pairs. During the 2015 breeding season on Praia Islet, six Madeiran wall lizards (Lacerta dugesii) were observed depredating a Monteiro's Storm-petrel chick about to fledge. In August 2015, a freshly killed chick without its head and four larger dead chicks were found. Eight other chicks had disappeared from their nests since our previous visit in early July. In 2016, there was further evidence of suspected lizard depredation. Three chicks were found predated and dragged out of their nests, and an additional 22 chicks disappeared from their nests before fledging. In 2016, Monteiro's Storm-petrel breeding success was only 26.8%, the lowest ever recorded. We suspect that some of the chick deaths and nest failures may be attributed to these introduced lizards. All chicks were found dragged out of their nests, a behavior that is not consistent with kills from aerial predators. Lizards were suspected because they are the only possible predator on land. Therefore, we recommend evaluating the impact of Madeiran wall lizards on the two Monteiro's Storm-petrel main colonies: Praia and Baixo Islets.