Organisms from all domains of life are infected by viruses, the most abundant microorganisms on earth. In Eukaryotes Serine/Threonine kinases play a central role in antiviral response. Bacteria however are not known to use protein phosphorylation as part of their defense against phages. In this study, we identified Stk2, a Serine/Threonine kinase that allows efficient immunity against bacteriophage infection in staphylococci. Upon phage infection, the kinase activity of Stk2 is activated by the phage protein PacK, leading to cell death, presumably through phosphorylation of proteins involved in essential cellular processes. Infected cells die before releasing infectious phages thereby protecting neighboring bacteria. This defense strategy belongs to a type of mechanistically diverse systems known as abortive infection (Abi). The presence of the phage DNA itself is not recognized by Stk2 but it appears that the Abi phenotype of Stk2 is triggered only during the lytic cycle of the phage. In S. aureus, a second Serine/Threonine kinase known as Stk1 or PknB, which is important for the cell-wall structure, antimicrobial resistance, and virulence, appears to be also involved in this anti-viral defense pathway and is required for efficient Stk2-mediated immunity. Stk2 is present in only some isolates of S. aureus and was noted in a few studies but its function remained unknown. Our work shows that mechanisms of viral defense that rely on protein phosphorylation constitute an anti-viral strategy conserved across domains of life.