We previously reported that Arabidopsis shows enhanced root hair density and length in response to cadmium (Cd). To understand how Cd stimulates root hair elongation, we examined the role of reactive oxygen species (ROS) and root hair elongation-related genes in root hair elongation/tip growth. In response to Cd, Arabidopsis displayed enhanced length and ROS levels in the root hairs, indicating ROS involvement. Furthermore, in response to Cd, ROS generation mutants, including root hair defective 2 (rhd2/rbohc), rbohh, rbohj, per1, per44 and per73, which have shorter root hairs because of impaired ROS generation, also showed increased root hair length and ROS levels, suggesting the involvement of these genes in ROS production in Cd-induced root hair elongation. Supporting the participation of ROS, N-acetyl-L-cysteine (ROS scavenger) treatment reduced root hair elongation and ROS levels. Additionally, the expression of proline-rich extension-like receptor kinase (PERK)5, PERK8, and root hair-specific 10 (RHS10), which are negative regulators of root hair elongation, was reduced by Cd and ROS (H₂O₂). Furthermore, the expression of expansin A7 (EXPA7) and expansin A18 (EXPA18), which are involved in root hair elongation, was remarkably induced by Cd and H₂O₂ in elongating root hairs and tips. Additionally, COBL9, which is involved in cell wall formation, was highly expressed in root hairs and tips in response to Cd and H₂O₂. Furthermore, changes in auxin and ethylene levels showed a similar pattern to that of root hair elongation and ROS at various concentrations of Cd, suggesting the involvement of auxin and ethylene in Cd/ROS-induced root hair elongation. Combined, it appears that Cd enhances root hair elongation/tip growth by elevating the expression of EXPA7, EXPA18 and COBL9 and by inhibiting the expression of PERK5, PERK8, and RHS10, which are ROS-mediated in Arabidopsis.