ABSTRACT Viral infections manipulate diverse post-translational modifications (PTMs), altering existing protein landscapes to create cellular environments favorable to replication. PTMs allow for rapid modulation of host environments by altering protein abundance, localization, and activity. Because of their dynamic nature, PTMs typify ideal immune response effectors or initiators. Our data identifies ADP-ribosylation, the modification of proteins with ADP-ribose (ADPr), as a rapid, immune-like response from infected cells that constitutes an antiviral response that counters influenza virus infection. Using cutting-edge ADPr-specific mass spectrometry approaches, we have characterized with single amino acid resolution the ADP-ribosylome during influenza virus infection and identified thousands of modifications on viral and host proteins. ADP-ribosylation has been associated with antiviral responses against multiple viruses. However, little is known about how viral infections trigger this response or the activiral mechanism(s) of ADP-ribosylation and poly(ADPr)-polymerases (PARPs), the enzymes that catalyze addition of ADPr to proteins. Here, we propose studies of ADP-ribosylation during influenza virus infection. We will identify the functional consequences of specific ADPr modifications, and building on our unique ADP-ribosylome dataset, investigate how ADP-ribosylation alters the function of specific viral and host proteins. We will interrogate the cellular pathways responsible for initiating ADP-ribosylation responses, defining the molecular triggers and PARPs that are activated during influenza virus infection. These experiments will elucidate the mechanisms of ADPr-mediated viral inhibition, the factors that trigger this response, and how viruses counter it, establishing ADP-ribosylation as a key aspect of cellular antiviral responses and perhaps as an entirely independent arm of antiviral defenses.