Non-coding RNAs are a class of RNAs with multiple roles in plant life. Covalently closed circular RNA molecules (circRNAs) have been recently shown to be a group of RNA isoforms that show widespread tissue-specific expression in plants, often cooperating with the corresponding linear mRNAs to regulate gene function. However, no previous study of poplar has identified circRNAs in the cambium and determined their potential roles in the cambium or xylem development. In the present study, we sequenced RNAs in the cambium of poplar seedlings at two developmental stages, and identified and characterized 4912 circRNAs. Alternative back-splicing circularization events for 87 genes were identified among the circRNAs derived from different chromosomes. A total of 1138 circRNAs originated from 928 host genes, which were classified among the three major functional categories by GO analysis. Thirty-nine circRNAs were differentially expressed between cambium samples of stems at two developmental stages. Twenty-four DEcircRNAs interacted with 98 miRNAs as targets, of which some were associated with cambium growth and development. The results suggest that circRNAs play important roles in the cambium in relation to the regulation of stem growth and development in poplar seedlings.