We developed a stochastic susceptible–exposed–infectious–removed (SEIR) model to simulate a range of plausible morbillivirus outbreak scenarios in a randomly mixing population of 170 endangered Hawaiian monk seals (Neomonachus schauinslandi). We then modeled realistic vaccination and quarantine measures to determine the potential efficacy of such mitigation efforts. Morbillivirus outbreaks represent substantial risk to monk seals—91% of simulated baseline outbreaks grew (R₀>1), and in one-third of the scenarios all, or nearly all, individuals were infected. Simulated vaccination efforts in response to an outbreak were not effective in substantially reducing infections, largely because of the prolonged interval between vaccination and immunity. Prophylactic vaccination, in contrast, could be an effective tool for preventing outbreaks. Herd immunity is practically achievable because of the small sizes of monk seal populations and the animals' accessibility on shore. Adding realistic spatial structure to the model, as informed by movement of seals tracked in the main Hawaiian Islands with the use of telemetry, greatly reduced the simulated impact of outbreaks (≤10 seals were infected in 62% of spatially structured simulations). Although response vaccination remained relatively ineffective, spatial segregation allowed herd immunity to be achieved through prophylactic vaccination with less effort. In a randomly mixing population of 170 seals, 86% would need to be vaccinated to achieve herd immunity in 95% of simulated outbreaks, compared to only approximately 60% in three spatially segregated subgroups with the same combined abundance. Simulations indicate that quarantining a modest number (up to 20) of ill seals has the potential to extinguish even fast-growing outbreaks rapidly. The efficacy of quarantine, however, is highly dependent upon rapid detection and response. We conclude that prophylactic vaccination combined with a quarantine program supported by vigilant surveillance and rapid, reliable diagnosis could greatly mitigate the threat of a morbillivirus outbreak in Hawaiian monk seals.