SUMMARY Root hair length (RHL) is an important character that affects nutrient acquisition in plants. The regulatory network in soybean controlling RHL is yet to be fully understood. In this study, we identified a quantitative trait locus (QTL) regulating RHL. One candidate causal gene in this QTL (GmbHLH113), preferentially expressed in root hairs, was annotated as encoding a basic helix–loop–helix transcription factor. In wild soybeans, the allelic type of GmbHLH113 with a glycine in the 13th residue, which was associated with a reduction in RHL, was shown to localize in the nucleus and activate gene transcription. Another allelic type with a single nucleotide polymorphism that resulted in a glutamate in the 13th residue is fixed in cultivated soybeans, and it lost the ability to localize to the nucleus or negatively regulate RHL. The ectopic expression of GmbHLH113 from W05 in Arabidopsis root hairs resulted in shorter RHL and reduced phosphorus (P) accumulation in shoots. Hence, a loss‐of‐function allele in cultivated soybeans might have been selected during domestication due to its association with a longer RHL and improved nutrient acquisition. Significance Statement The genetic mechanism behind root hair length (RHL) regulation in soybean is not yet fully understood. This study functionally characterized a novel negative regulator of RHL in wild soybeans and a loss‐of‐function allelic type fixed in cultivated soybeans resulting in longer root hairs and possibly more efficient nutrient acquisition, suggesting this loss‐of‐function allele could have been indirectly selected during soybean domestication.