Stem strength represents one of the most critical agronomic traits, as it enables plants to resist lodging and bending, thereby contributing to their overall yield and quality. Various transcription factors have been shown to regulate stem strength in crops; however, the mechanisms underlying stem strength formation and regulation remain largely unexplored, particularly in ornamental plants. This study identified a group IIe WRKY transcription factor PlWRKY29 in herbaceous peony. Tobacco plants overexpressing PlWRKY29 exhibited significantly enhanced stems, expanded xylem, thickened cell walls and elevated lignin content compared to wild-type specimens. Conversely, PlWRKY29-silenced herbaceous peony demonstrated opposite characteristics. Further investigation of the regulatory mechanism revealed that PlWRKY29 bound to the promoter of PlLAC15, which encodes a monolignol polymerization gene that facilitates lignin deposition. These findings demonstrate that PlWRKY29 positively regulates lignin biosynthesis and stem strength, advancing our understanding of lignin biosynthetic regulation in plants.