Marine clay mineral authigenesis, referred to as reverse (silicate) weathering, is one of the first-order controls on seawater pH through the generation of acidity and thus plays a significant role in controlling carbon cycling between marine sediments, oceans and the atmosphere over geological timescales. Reverse weathering is mainly regulated by the rates of silicate and carbonate weathering on the continents, the reactivity of detritus supplied to the oceans and the rates of seafloor weathering. These processes provide essential dissolved components (e.g. K+, Mg2+, Ca2+, Si(OH)4, Al3+, Fe2+/3+) to the marine porewater inventory that cause authigenic clay minerals, such as odinite, glauconite, celadonite and greenalite, to form close to the sediment–seawater interface. Such clay mineral reactions impact the sedimentary cycling versus sequestration of chemical elements, importantly Si, Fe, Mg and K, and consequently contribute to the fluctuations in climate and seawater composition recorded in marine archives over geological time. This review explores the links between reverse silicate weathering and the climate system across geological timescales and provides estimates of the elemental uptake fluxes associated with modern-day clay mineral authigenesis. Novel isotope proxies (e.g. δ41K and δ30Si) and promising new dating techniques (e.g. in situ Rb/Sr geochronology) provide improved constraints on the timing, kinetics and environmental significance of clay mineral reactions on the ocean floor. We also consider recent geoengineering developments linked to reverse weathering reactions, such as ongoing attempts to reduce atmospheric CO2 concentrations via marine alkalinity enhancement and the application of marine clay mineral-based slow-release fertilizers to soils to optimize nutrient availability.