Abstract Rainfall among the Amazon has been shown to decline sharply once forest loss exceeds a critical threshold, typically around 30%–40%. However, future interactions between climate change and deforestation on this threshold remain unclear. We model their combined effects by 2050 on rainfall and temperature. Our findings indicate that climate and land‐use change lower the deforestation threshold for rainfall decline—from 50% forest loss under land‐use change alone to 45% under SSP1‐2.6 and 10% under SSP5‐8.5 (90 × 90 km reference grids). Land‐use change alone is projected to warm the region by 0.29°C, while the combined effects under SSP5‐8.5 lead to 1.4°C warming. Annual rainfall is projected to decline by 1.7% due to land‐use change alone and up to 13.9% under SSP1‐2.6. Future climates amplify the importance of forests in maintaining moisture supply for rainfall. These findings underscore the growing vulnerability of the Southern Amazon to rainfall disruption and reduced land–atmosphere system resilience.