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Stems are prone to bending or lodging due to inadequate stem strength, which seriously reduces the cut-flower ornamental quality of herbaceous peony (<i>Paeonia lactiflora</i> Pall.). Plant LACCASE (LAC), a copper-containing polyphenol oxidase, has been shown to participate in the polymerization process of monolignols; however, the role of <i>LAC</i> in regulating the stem strength of <i>P. lactiflora</i> remains unclear. Here, the full-length cDNA of <i>PlLAC15</i>, which demonstrated a positive association with stem strength, was isolated. It consisted of 1790 nucleotides, encoding 565 amino acids that had four typical laccase copper ion-binding domains. Moreover, <i>PlLAC15</i> was highly expressed in the stem, and its expression level gradually significantly increased during stem development. Furthermore, PlLAC15 was found to be localized specifically to the cell wall, and its recombinant protein exhibited laccase activity. Additionally, the role of <i>PlLAC15</i> in regulating the stem strength of <i>P. lactiflora</i> was confirmed by transgenic studies. When <i>PlLAC15</i> was overexpressed in tobacco, stem strength increased by more than 50%, S-lignin was significantly deposited, and the lignification degree of stem xylem fiber cells increased. These results suggested that <i>PlLAC15</i> facilitated S-lignin deposition to enhance stem strength in <i>P. lactiflora</i>, which would provide precious information that benefits future exploration of stem bending or lodging resistance in plants.