Ubiquitination is essential for numerous eukaryotic cellular processes. Here, we show that the type III effector CteC from Chromobacterium violaceum functions as an adenosine diphosphate (ADP)-ribosyltransferase that specifically modifies ubiquitin via threonine ADP-ribosylation on residue T66. The covalent modification prevents the transfer of ubiquitin from ubiquitin-activating enzyme E1 to ubiquitin-conjugating enzyme E2, which inhibits subsequent ubiquitin activation by E2 and E3 enzymes in the ubiquitination cascade and leads to the shutdown of polyubiquitin synthesis in host cells. This unique modification also causes dysfunction of polyubiquitin chains in cells, thereby blocking host ubiquitin signaling. The disruption of host ubiquitination by CteC plays a crucial role in C. violaceum colonization in mice during infection. CteC represents a family of effector proteins in pathogens of hosts from different kingdoms. All the members of this family specifically ADP-ribosylate ubiquitin. The action of CteC reveals a new mechanism for interfering with host ubiquitination by pathogens. Display omitted •C. violaceum blocks host ubiquitination via the type III effector CteC•CteC is an ADP-ribosyltransferase that specifically modifies Ub on T66•Threonine ADP-ribosylation of Ub by CteC disrupts host ubiquitin signaling•CteC represents a family of bacterial effector proteins that ADP-ribosylate ubiquitin CteC from Chromobacterium violaceum represents a family of bacterial effector proteins that harbors adenosine diphosphate (ADP)-ribosyltransferase activities and specifically modifies ubiquitin on residue T66, which causes the shutdown of polyubiquitin synthesis and disrupts the recognition and reversal of polyubiquitin in host cells.