Hydrothermal particles originate in buoyant plumes emanating from seafloor thermal vents1–4 and accumulate as metalliferous sediments found along oceanic spreading centres5–7. Observational evidence of the transport pathway of hydrothermal particles is scarce, however, and the structure, extent and particle concentrations of hydrothermal plumes remain largely conjectural. To evaluate the potential of hydrothermal plumes as particle transport agents, we mapped their distribution along and across the 80–100-m-deep axial valley of the southern Juan de Fuca Ridge in the north-east Pacific Ocean (∼44°30′ N, 130° W), an area of known hydrothermal activity8–11. Continuous light-scattering measurements, reported here, defined numerous particle plumes centred 30–120 m above bottom (m.a.b.); elemental analysis of the plume particles and the theta (potential temperature)–S (salinity) signatures of the plume water verified their hydrothermal origin. Concentrations of hydrothermal particles remained elevated at ridge-crest depths at least 100 km from the ridge, indicating distant off-axis dispersal of hydrothermal emissions.