<p>Understanding the controls of rainfall isotopic composition (δ¹⁸O_p) is essential to interpret isotopic paleoclimate records, yet it remains challenging in areas with complex climate and varied topography, such as northern Central America. Here, we present two years of daily rainfall isotope measurements from Guatemala to evaluate the regional and local climatic controls on δ¹⁸O_p variability. We show that regional-scale convection and upstream rainout drive δ¹⁸O_p variability and integrate multi-annual model-based δ¹⁸O_p data supporting a persistent regional control across multiple timescales, from daily to interannual. Further, we document a consistent link between higher rainfall ¹⁸O depletion associated with both more intense regionally-organized convective systems and larger stratiform rain fractions. Moisture source analysis shows a dominant contribution from the Caribbean–Atlantic, and secondarily from the Mesoamerican continent, Gulf of Mexico, and Eastern Pacific. While changes between dominant source regions are associated with distinct δ¹⁸O_p signatures, back-trajectory analysis and a source-based isotope reconstruction demonstrate a larger upstream airmass rainout control on the seasonal variations of δ¹⁸O_p. Our results suggest that δ¹⁸O_p in Guatemala rainfall are sensitive to regionally-integrated hydroclimatic processes, and that isotope-based paleoclimate records from northern Central America can provide reconstructions of past changes in regional-scale atmospheric convection and upstream rainfall amount.</p>