The expansion of land plants in the Devonian is thought to have played a key role in regulating global climate and the carbon cycle, but the mechanisms of this linkage remain opaque. Here, we report new carbonate δ^44/40Ca, δ²⁶Mg, δ⁷Li and ⁸⁷Sr/⁸⁶Sr data through two uppermost Devonian carbonate successions to evaluate changes in continental weathering and seawater chemistry that were potentially triggered by the spread of seeds plants, to assess their role in the initial cooling phase of the Late Paleozoic Ice Age. The model based on δ^44/40Ca and δ²⁶Mg helps constrain diagenetic overprints during early marine burial. Mass-balance calculations of seawater δ⁷Li and ⁸⁷Sr/⁸⁶Sr quantify an increase of ~75-100% in continental physical erosion rates relative to chemical weathering, implicating an enhanced hydrological cycle in this climatic transition. Numerical simulations using the coupled Community Earth System Model (CESM 1.2.2) support stronger precipitation and surface runoff on continents with vegetated interiors (Scenario VEG) compared to those with non-to-less vegetated interiors (Scenario NOVEG). Thus, we hypothesize that rapid expansion of seed plants into continental interiors during the latest Devonian resulted in intensification of the hydrological cycle, increasing riverine nutrient input to oceans and further stimulating the oceanic eutrophication and anoxia that triggered a series of Devonian biotic crises.