The mechanisms by which viruses hijack their host’s genetic machinery are of current interest. When bacteriophage T4 infects Escherichia coli , three different ARTs (ADP-ribosyltransferases) reprogram the host’s transcriptional and translational apparatus through ADP-ribosylation using nicotinamide adenine dinucleotide (NAD) as substrate 1,2 . Recently, NAD was identified as a 5’-modification of cellular RNAs 3–5 . Here, we report that T4 ART ModB accepts not only NAD but also NAD-capped RNA (NAD-RNA) as substrate and attaches entire RNA chains to acceptor proteins in an “RNAylation” reaction. ModB specifically RNAylates ribosomal proteins rS1 and rL2 at defined arginine residues, and a specific group of E. coli and T4 phage RNAs is linked to rS1 in vivo . T4 phages that express an inactive mutant of ModB show a decreased burst size and slowed lysis of E. coli . Our findings reveal a distinct biological role of NAD-RNA, namely activation of the RNA for enzymatic transfer to proteins. The attachment of specific RNAs to ribosomal proteins might provide a strategy for the phage to modulate the host’s translation machinery. This work exemplifies the first direct connection between RNA modification and post-translational protein modification. As ARTs play important roles far beyond viral infections 6 , RNAylation may have far-reaching implications.