Quercus mongolica, a dominant species in northern China, has shown notable range expansion despite intensifying drought. Although highly drought-tolerant, its adaptive mechanisms across geographic drought gradients remain unclear. We sampled 14 populations across a drought gradient to examine leaf morphological adaptations and their implications for water transport and photosynthesis. Leaf area, length, width, and perimeter declined significantly with increasing aridity (p < 0.05). Meanwhile, leaf allometry shifted adaptively, with increased length-to-width ratios and specific leaf area, yielding narrower, elongated leaves. These structural traits were coupled with higher vein density and light capture, suggesting coordinated hydraulic and photosynthetic adaptation. Changes in leaf size, specific leaf area, and vein density emerged as key drought adaptations. These findings underscore the role of trait plasticity in supporting Q. mongolica’s dominance under drought.