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The extent to which ambient seawater permeates and interacts with submarine lithologies (e.g., sediments and seafloor basalt) is a critical constraint on the timing and rate of in situ subseafloor silicate weathering. Oxygen isotope (δ18O) values of low-temperature silicate minerals in the marine record often present seemingly inconsistent oceanographic and diagenetic histories. These uncertainties largely arise because neither formation temperature (e.g., burial or alteration) nor the modification of seawater δ18O values through water–rock interactions are well constrained. However, the triple oxygen isotope (Δ17O) approach provides additional constraints on the diagenetic temperature and seawater modification. Here we present two case studies of Δ17O variations in low-temperature silicate minerals from widely disparate marine settings that represent potential endmembers in subseafloor diagenetic environments and seawater modification through water–rock interactions. Marine sediment cores from the Ross Sea, Antarctica, collected during IODP Expedition 374 and the ANDRILL McMurdo Ice Shelf program, contain well-preserved biogenic opal (diatomite) of Pleistocene to middle Miocene age (~2.2–16.5 Ma). The mineral structure of opal from these sites (IODP U1521 and U1523; AND-1b and AND-2a) ranges from opal-A to chert, and the Δ17O values reflect isotopic equilibrium with a significantly modified seawater at a range of temperatures consistent with the geothermal gradient and burial depth. Measured Δ17O values for all opal samples fall below the seawater equilibrium curve and likely reflect equilibration with pore waters. The abundance of hydrous mineral phases (e.g., mirabilite, authigenic clays) in the Ross Sea cores suggest that water-rock interactions may have altered the pore water Δ17O values. Pore water δ18O values and chemistry at the ANDRILL sites suggest the presence of a cryogenic brine with a low δ18O value; however, in the IODP sites on the continental shelf and slope, pore water δ18O values are closer to that of modern Ross Sea Bottom Water. In a very different geologic setting in the North Atlantic, similarly modified seawater Δ17O values are recorded in alteration minerals (e.g., celadonite, saponite, and zeolite) in submarine basalt veins/vesicles from IODP Site U1564, which is located east of the Reykjanes Ridge in ~32.4 Ma crust. The alteration minerals Δ17O values appear to show a mixing relationship between seawater and unaltered basalt endmember with varying water–rock ratios and/or formation temperatures, which suggests fluid evolution or mixing of fluids with different Δ17O values. Observed Δ17O values in ancient geologic materials (e.g. Archean cherts) have been interpreted as reflecting primary oceanographic conditions or subsequent diagenetic alteration by meteoric waters. In the geologic settings described here, the Δ17O variability appears to record significant in situ subseafloor modification of seawater oxygen isotope values through low-temperature water–rock interactions. Constraining the timing and extent of water–rock interactions are, therefore, essential for refining models of geochemical reactions, fluid flow, global element cycling, and deep-biosphere microbial processes in the marine subseafloor environment.