Abstract <p>Dehydrins (DHNs) belong to the second family of Late Embryogenesis Abundant (LEA) proteins. DHNs are hydrophilic proteins ubiquitously expressed in plants under various abiotic stresses, particularly drought. However, the function of DHN in response to drought stress in herbaceous peony (<i>Paeonia lactiflora</i> Pall.) has not been reported. Here, we isolated <i>PlRab18</i>, which encodes a 148-amino-acid protein (15.496 kDa), and classified it as a Y₂SK₂-type dehydrin based on the number and composition of conserved domains. Furthermore, phylogenetic analysis showed that herbaceous peony has a close relationship with Rab18 members of the dehydrin family in other species. Quantitative real-time PCR (qRT-PCR) analysis revealed that drought stress significantly induced <i>PlRab18</i> expression. Subcellular localization analysis revealed that PlRab18 was localized in both the nucleus and cell membrane. Virus-induced gene silencing (VIGS) of <i>PlRab18</i> reduced drought tolerance in herbaceous peony. Compared to the control group, the <i>PlRab18</i>-silenced plants exhibited an approximately 15% decrease in leaf water content, accompanied by a significant increase in both relative electrical conductivity (REC) and malondialdehyde (MDA) content. Photosynthetic efficiency was compromised in the <i>PlRab18</i>-silenced plants, as indicated by decreases in the net photosynthetic rate, stomatal conductance, and transpiration rate. In addition, VIGS revealed severe oxidative damage in the <i>PlRab18</i>-silenced plants, as characterized by substantial accumulation of reactive oxygen species (ROS) and a marked decline in antioxidant enzyme activities. These results indicated that <i>PlRab18</i> enhanced drought tolerance in herbaceous peony under water deficit. This study contributes to a better understanding of the drought-resistant function of <i>DHN</i> in herbaceous peony and identify a potential candidate gene for breeding drought-resistant cultivars.</p>