A few moderately long-term studies have documented population dynamics of the invasive Hemigrapsus sanguineus (Asian Shore Crab) and species with which it interacts. One such study on Cape Cod reported exponential growth of the Asian Shore Crab through 2012, concurrent with declines in 2 resident species, but recent data at nearby sites suggest considerable geographic variation in population dynamics, with modest recovery of resident crab populations. We monitored the Cape Cod population for an additional 5 years to determine whether population growth of the Asian Shore Crab had slowed and whether there was any change in population dynamics of Carcinus maenas (Green Crab), Littorina littorea (Common Periwinkle), or Mytilus edulis (Blue Mussel). Asian Shore Crab density declined by nearly 90% since 2012. There was no evidence of recovery by the Green Crab, but the Blue Mussel experienced a brief but substantial increase in density in 2015, when both crab species displayed >70% reductions in density from the previous year. The pattern of population dynamics of the Asian Shore Crab from 2003 to 2017 is consistent with boom–bust dynamics, but it remains unclear whether the population will equilibrate near its current density or undergo recurrent boom–bust cycles.