The ratio of strontium isotopes, ⁸⁷Sr/⁸⁶Sr, in seawater is homogenous at any given time, yet varies considerably throughout the geological record. This variation is thought to stem from changes in the balance of predominantly radiogenic Sr entering the oceans via dissolved riverine transport, and unradiogenic Sr sourced from mid-ocean ridge hydrothermal activity. Recent evidence suggests, however, that hydrothermal exchange at mid-ocean ridges is a factor of 3 too low to balance Sr added to the oceans from dissolved continental riverine fluxes. Here we present evidence that the arrival and subsequent dissolution of riverine particulate material in seawater is a significant contributor of both radiogenic and unradiogenic Sr to the oceans. Batch experiments demonstrate that between 0.15% and 27.36% of Sr is liberated from riverine particulates to seawater within 6 months. The rates of release are dependent on surface area and particulate composition, with volcanic riverine material more reactive than continental riverine particulates. The observed rapid Sr release rate from riverine particulate material has important consequences for both chemical and isotopic mass balances in the ocean and the application of the ⁸⁷Sr/⁸⁶Sr weathering proxy to the geological record. The dissolution of riverine particulate material is likely, based on these findings, to at least partially account for the imbalance between Sr sources to the oceans.