A weak alkaline condition (pH > 8) is a general requirement for oxidative self-polymerization of dopamine. Here, we first demonstrated the generation of polydopamine in an acidic environment via a hydrothermal method. The pH scope of self-polymerization of dopamine is extended to pH similar to 1 in a hydrothermal process. Polydopamine generated via a hydrothermal method shows similar chemical features and radical scavenging activity with that generated in a basic environment.