Prp19 is an essential splicing factor and a member of the U-box family of E3 ubiquitin ligases. Prp19 forms a tetramer via a central coiled-coil domain. Here, we show the U-box domain of Prp19 exists as a dimer within the context of the Prp19 tetramer. A high-resolution structure of the homodimeric state of the Prp19 U-box was determined by X-ray crystallography. Mutation of the U-box dimer interface abrogates U-box dimer formation and is lethal in vivo. The structure of the U-box dimer enables construction of a complete model of Prp19 providing insights into how the tetrameric protein functions as an E3 ligase. Finally, comparison of the Prp19 U-box homodimer with the heterodimeric complex of BRCA1/BARD1 RING-finger domains uncovers a common architecture for a family of oligomeric U-box and RING-finger E3 ubiquitin ligases, which has mechanistic implications for E3 ligase-mediated polyubiquitination and E4 polyubiquitin ligases.