The American lobster Homarus americanas shares rocky shore habitats and resources with two common invasive crab species in the northwest Atlantic: the green crab Carcinus maenas and the Asian shore crab Hemigrapsus sanguineus. Juvenile or early benthic phase (EBP) lobsters live under the same boulders as these crabs and are potentially vulnerable to crab predation and competition. Time-lapse photography was used to quantify aggressive interactions between EBP lobsters and invasive crabs in the laboratory. Lobsters behaved far more aggressively toward Asian shore crabs (initiating 11.8 attacks/h) than green crabs (4.9 attacks/h). Crab behavior varied widely as well, as green crabs were five times more likely to attack EBP lobsters than were Asian shore crabs. Thus, lobsters initiated 75.5% of aggressive interactions with Asian shore crabs, compared with only 22.3% with green crabs. Invasion history was a possible reason for this disparity, as green crabs have shared geographic range with American lobsters for more than 100 y. compared with only more than 10 y for Asian shore crabs. Differences in lobster responses to the two crab species suggest that aggressive behavior is the default for EBP lobsters and that they attacked the novel species (H. sanguineus) because they had not developed avoidance behavior to the potential crab threat. Green crabs posed more of a predation threat than Asian shore crabs, consuming over 80% of lobsters in 24-h trials. These results suggest that invasion history and predator—prey adaptation may shape the behavior and ecological interactions of native and invasive species.