Pacific decadal variability (PDV) causes widespread, persistent fluctuations that affect climate, water resources, and fisheries throughout the Pacific basin, yet the magnitude, frequency, and causes of PDV remain poorly constrained. Here we present an absolutely dated, subannually resolved, 446 yr stable oxygen isotope (δ18O) cave record of rainfall variability in Vanuatu (southern Pacific Ocean), a location that has a climate heavily influenced by the South Pacific Convergence Zone (SPCZ). The δ18O-based proxy rainfall record is dominated by changes in stalagmite δ18O that are large (∼1‰), quasi-periodic (∼50 yr period), and generally abrupt (within 5–10 yr). These isotopic changes imply abrupt rainfall changes of as much as ∼1.8 m per wet season, changes that can be ∼2.5× larger than the 1976 C.E. shift in rainfall amount associated with a PDV phase switch. The Vanuatu record also shares little commonality with previously documented changes in the Intertropical Convergence Zone during the Little Ice Age or solar forcing. We conclude that multidecadal SPCZ variability is likely of an endogenous nature. Large, spontaneous, and low-frequency changes in SPCZ rainfall during the past 500 yr have important implications for the relative magnitude of natural PDV possible in the coming century.