Ocean acidification refers to the lowering of the ocean’s pH due to the uptake of anthropogenic CO₂ from the atmosphere. Coral reef calcification is expected to decrease as the oceans become more acidic. Dissolving calcium carbonate (CaCO₃) sands could greatly exacerbate reef loss associated with reduced calcification but is presently poorly constrained. Here we show that CaCO₃ dissolution in reef sediments across five globally distributed sites is negatively correlated with the aragonite saturation state (Ω_ar) of overlying seawater and that CaCO₃ sediment dissolution is 10-fold more sensitive to ocean acidification than coral calcification. Consequently, reef sediments globally will transition from net precipitation to net dissolution when seawater Ω_ar reaches 2.92 ± 0.16 (expected circa 2050 CE). Notably, some reefs are already experiencing net sediment dissolution.