A gene family is formed by duplication of a single original gene. Poplar trees (genus Populus) are important, principally because of their ecological and economic benefits, and are one of the most widely distributed and adaptable trees in the world. Systematic identification and annotation of gene family members are primary steps in studying the function and evolution of poplar genomes. Here, we describe the construction of the Gene Family Database in Poplar (GFDP), which contains information that systematically describes 6551 genes distributed in 145 gene families. GFDP is designed to present important biological information, such as gene structure, protein length, isoelectric point and functional and evolutionary information, using highly visual displays. Data and graphs are visualized by a web-based interface. Users can browse and download data through all the major browsers. GFDP provides a comprehensive platform with a solid foundation for further study of poplar gene families. GFDP is free available.