REALLY INTERESTING NEW GENE (RING) proteins play important roles in the regulation of many processes by recognizing target proteins for ubiquitination. Previously, we have shown that the expression of PtaRHE1, encoding a Populus tremula x P. alba RING-H2 protein with E3 ubiquitin ligase activity, is associated with tissues undergoing secondary growth. To further elucidate the role of PtaRHE1 in vascular tissues, we have undertaken a reverse genetic analysis in poplar. Within stem secondary vascular tissues, PtaRHE1 and its corresponding protein are expressed predominantly in the phloem. Down-regulation of PtaRHE1 in poplar by artificial miRNA triggers alterations in phloem fiber patterning characterized by an increased portion of secondary phloem fibers that have a reduced cell wall thickness and a change in lignin composition with lower amounts of syringyl units as compared to wild type plants. Following a RNA-seq analysis, a biological network involving hormone, stress signalling as well as developmental processes could be delineated. Several candidate genes possibly associated with the altered phloem fiber phenotype observed in amiRPtaRHE1 poplar were identified. Altogether, our data suggest a regulatory role for PtaRHE1 in secondary phloem fiber development.