Characterizing variability in the global water cycle is fundamental to predicting impacts of future climate change; understanding the role of the Pacific Walker circulation (PWC) in the regional expression of global water cycle changes is critical to understanding this variability. Water isotopes are ideal tracers of the role of the PWC in global water cycling because they retain information about circulation-dependent processes including moisture source, transport, and delivery. We collated publicly available measurements of precipitation 𝛿18O (𝛿18O𝑃) and used novel data processing techniques to synthesize long (34 yr), globally distributed composite records from temporally discontinuous 𝛿18O𝑃 measurements. We investigated relationships between global-scale 𝛿18O𝑃 variability and PWC strength, as well as other possible drivers of global 𝛿18O𝑃 variability—including El Niño–Southern Oscillation (ENSO) and global mean temperature—and used isotope-enabled climate model simulations to assess potential biases arising from uneven geographical distribution of the observations or our data processing methodology. Covariability underlying the 𝛿18O𝑃 composites is more strongly correlated with the PWC (r = 0.74) than any other index of climate variability tested. We propose that the PWC imprint in global 𝛿18O𝑃 arises from multiple complementary processes, including PWC-related changes in moisture source and transport length, and a PWC- or ENSO-driven "amount effect" in tropical regions. The clear PWC imprint in global 𝛿18O𝑃 implies a strong PWC influence on the regional expression of global water cycle variability on interannual to decadal time scales, and hence that uncertainty in the future state of the PWC translates to uncertainties in future changes in the global water cycle.