<p>This study provides an integrated assessment of long-term trends in extreme climate indicators and land-use and land-cover trajectories across the Iguaçu Triple Frontier, a climatically sensitive and socioeconomically dynamic region shared by Argentina, Brazil, and Paraguay. This work examines a transboundary sector of South America that lacks integrated climate–land studies. Daily surface observations from 1961 to 2023 were used to validate ERA5-Land through Willmott’s d, Pbias, MAE, and RMSE, and the validated dataset supported ETCCDI-based analyses using the Mann–Kendall test, Sen’s slope, and Pettitt, SNHT, and Buishand tests to identify trends and structural changes. Annual MapBiomas data from 1985 to 2023 were evaluated with the same statistical procedures to detect temporal discontinuities in LULC trajectories. LULC–climate relationships were examined at the municipality scale using aggregated LULC classes and spatially averaged ERA5-Land indices. The region shows pervasive warming, most evident in the rise of minimum temperatures and the contraction of the diurnal temperature range. Spring displays the strongest intensification, with increases in hot-day and warm-night indices and declines in cold extremes. Precipitation trends are more heterogeneous, with October increases, shorter wet spells, and nonstationarity in March, August, and October. LULC patterns reveal rapid urbanization and forest recovery in Brazil, deforestation and cropland expansion in Paraguay, and native forest loss offset by silviculture in Argentina. Correlation analyses indicate that urban, silvicultural, and bare surfaces are consistently associated with warmer and drier conditions, whereas forests, pastures, wetlands, and water bodies moderate extremes. These results indicate that climatic changes and land-use transitions jointly shape the region’s thermal and hydrological conditions, highlighting the need for basin-scale coordination.</p> Graphical Abstract <p>This graphical abstract provides a concise and visually organized summary of the study “Warming, Rainfall Extremes, and Land-Use Transformation in the Iguaçu Triple Frontier, South America”. The figure summarizes the main methodological steps and key findings, illustrating climatic indicators and land-use transitions in a simplified visual format. The objective, placed at the top, highlights the analysis of long-term ETCCDI-based climate extremes and Land-Use/Land-Cover (LULC) trajectories across Argentina, Brazil, and Paraguay, emphasizing their interactions between 1985 and 2023. In the Methodology section, data sources are depicted by a weather-station icon for surface observations and ERA5-Land validation (Willmott’s d, Pbias, MAE, RMSE) and a map/tree icon for annual MapBiomas LULC data (1985–2023). Analytical methods—ETCCDI indices, Mann–Kendall, Sen’s slope, and homogeneity tests (Pettitt, SNHT, Buishand)—are represented by a trend-graph and satellite symbol, underscoring the use of Google Earth Engine (GEE) and QGIS. The Key Results section illustrates region-wide, year-round warming (TMAX, TMIN, TXx, TX90p, TN90p, WSDI↑), shifts in precipitation patterns (CWD↓, R95p↑, JAS drying, October wetting, Brazil’s December RX1day↓ and SDII↓), and a major 2000–2005 land-use transition: urbanization and forest recovery in Brazil, deforestation and soybean expansion in Paraguay, and silvicultural replacement of native forests in Argentina. The Conclusion conveys that built, silvicultural, and non-vegetated surfaces amplify warming and drying, whereas forests, pastures, wetlands, and water bodies mitigate these effects. Together, these processes define a shared thermal and hydrological pattern across borders and emphasize the urgency of integrated basin-scale governance for climate adaptation and socio-environmental resilience.</p> <p></p>