Westerly winds from the Choco low-level jet direct moisture into the Western Cordillera of northern South America, where subsequent oro- graphic lifting creates the wettest region in the world. While the Choco jet is projected to weaken by the end of the 21st century, climate models show considerable disagreement about the extent of jet weakening. Using contemporary observations we demonstrate that the configuration of the Choco jet is captured by hydrogen isotopes in precipitation (δDp). As the Choco jet increases in strength, δDp increases, and as the Choco jet weakens δDp decreases. We apply this framework to a new record of reconstructed δDp using leaf waxes in ocean sediments off the coast of Ecuador that spans the Plio-Pleistocene. Low δDp in the early Pliocene indicates a weakened Choco jet in a warmer climate state, which is at- tributed to a low meridional sea surface temperature gradient between the equatorial and off-equatorial eastern tropical Pacific. Near the end- Pliocene (~3 Ma), the Choco jet abruptly weakens, possibly from shifts in the Intertropical Convergence Zone, forced by high latitude Northern Hemisphere cooling. In complementary isotope-enabled climate simulations, a weakened Choco jet in the early Pliocene is matched by a decrease in δDp and hydroclimate changes in northwestern South America. Precipitation from the Choco jet can cause deadly landslides and a weakened Choco jet in the early Pliocene implies a southward shift of these hazards along the Pacific coast of South America in the future.