Seasonally dry tropical forests, like the Caatinga, exhibit natural precipitation seasonality, influencing water sources for woody vegetation. This study assessed how seasonality affects the water dynamics of five deciduous plant species in the Caatinga using isotopic analyses (δ ²H and δ ¹⁸O) of xylem water, soil, and precipitation across wet, intermediate, and dry periods. During the wet season, plant isotopic signals were more enriched than those of soil, with overlapping dual-space isotopic lines. Despite differences between plant and local meteoric water lines (LMWL), δ ²H mean values showed no significant variation, indicating partial reliance on rainwater and soil. In the dry season, isotopic signals remained enriched, but plant and soil isotopic lines diverged, reflecting shifts in water sources. Some individuals accessed shallow soil water layers, evidenced by comparable δ ¹⁸O mean values. The study revealed significant variability in water source dynamics across seasons, with vegetation primarily relying on shallow soil water and precipitation during wet periods. However, the exact water sources accessed remain inconclusive due to methodological limitations. These findings underscore the complexity of water dynamics in the Caatinga and emphasize the need for refined methodologies to improve understanding of vegetation water-use strategies. This knowledge is crucial for informing conservation strategies in this unique biome, particularly as it faces increasing pressure from climate change and human activities.