AbstractBacteriophages play critical roles in the biosphere, but their vast genomic diversity has obscured their evolutionary origins, and phylogenetic analyses have traditionally been hindered by their lack of universal phylogenetic marker genes. In this study we mine metagenomic data and identify a clade of Caudovirales that encodes the β and β′ subunits of multi-subunit RNA polymerase (RNAP), a high-resolution phylogenetic marker which enables detailed evolutionary analyses. Our RNAP phylogeny revealed that the Caudovirales RNAP forms a clade distinct from cellular homologs, suggesting an ancient acquisition of this enzyme. Within these multimeric RNAP-encoding Caudovirales (mReC), we find that the similarity of major capsid proteins and terminase large subunits further suggests they form a distinct clade with common evolutionary origin. Our study characterizes a clade of RNAP-encoding Caudovirales and suggests the ancient origin of this enzyme in this group, underscoring the important role of viruses in the early evolution of life on Earth.