Wood is the most abundant biomass in perennial woody plants and is mainly made up of secondary cell wall. R2R3-MYB transcription factors are important regulators of secondary wall biosynthesis in plants. In this study, we describe the identification and characterization of a poplar MYB transcription factor PtoMYB92, a homolog of Arabidopsis MYB42 and MYB85, which is involved in the regulation of secondary cell wall biosynthesis. PtoMYB92 is specifically expressed in xylem tissue in poplar. Subcellular localization and transcriptional activation analysis suggest that PtoMYB92 is a nuclear-localized transcriptional activator. Overexpression of PtoMYB92 in poplar resulted in an increase in secondary cell wall thickness in stems and ectopic deposition of lignin in leaves. Quantitative real-time PCR showed that PtoMYB92 specifically activated the expression of lignin biosynthetic genes. Furthermore, transient expression assays using a β-glucuronidase (GUS) reporter gene revealed that PtoMYB92 is an activator in the lignin biosynthetic pathway during secondary cell wall formation. Taken together, our results suggest that PtoMYB92 is involved in the regulation of secondary cell wall formation in poplar by controlling the biosynthesis of monolignols.