Amazonian forests harbor a large variety of understory herbs adapted to areas with different hydrological conditions, ranging from well‐drained to seasonally flooded forests. The presence versus absence of flooding forms the extremes of a hydrological gradient, with various intermediate conditions, such as seasonal soil waterlogged areas, in between. We investigated the relationship between understory herbs and hydrological conditions in Central Amazonian forests using eighty‐eight 250 × 2 m plots distributed along a 600‐km transect. Hydrological conditions were determined regionally by precipitation and locally by topographic conditions based on drainage potential, flooding height and soil permeability (sand content). Soil cation concentration was used as a proxy for soil fertility. The floristic dissimilarities among plots were visualized by Nonmetric Multidimensional Scaling, and simple and multiple regressions were used to identify the best predictor of herb species composition. Local drainage potential was more important in determining herb species composition than soil fertility or precipitation at non‐flooded and flooded sites. Flooded sites comprised a very distinctive herb species composition even when the flood height was low (0.3 m). We conclude that hydrological conditions are the primary constraint of herb distribution within this flat regional landscape with moderate amounts of soil fertility variation (0.09–2.280 cmol(+)/kg). Hydrological models that consider local water conditions explained the largest part of herb species composition. Therefore, predictions of species distribution based on large‐scale climatic variables may underestimate the favorable area for understory herbs if the variation on local hydrological conditions is not considered.