Brassinosteroids (BRs) play important roles in many aspects of plant growth and development. BR-induced AtBEE3 (brassinosteroid enhanced expression 3) is required for a proper BR response in Arabidopsis. Here, we identified a poplar (Populus alba x P. glandulosa) BEE3 homolog encoding a putative basic helix-loop-helix (bHLH)-type transcription factor through microarray analysis. Transcripts of PagBEE3 were mainly detected in stems, with the internode having a low level of the transcripts and the node having a relatively higher level. The function of the PagBEE3 gene was investigated through the phenotypic analyses with PagBEE3-overexpressing (ox) transgenic lines. This work mainly focused on a potential role of PagBEE3 in stem growth and development of polar. The PagBEE3-ox poplar showed thicker and longer stems than wild-type plants. The xylem cells from the stems of PagBEE3-ox plants revealed remarkably enhanced proliferation, resulting in an earlier thickening growth than wild-type plants. Microarray analysis revealed that the expression of many genes involved in xylem cell proliferation and development was altered in the PagBEE3-ox plants. Therefore, this work suggests that xylem development of poplar is accelerated in PagBEE3-ox plants and PagBEE3 plays a role in the stem growth by increasing the proliferation of xylem cells to promote the initial thickening growth of poplar stems.