Phase separation is a vital mechanism that mediates the formation of biomolecular condensates and their functions. Necroptosis is a lytic form of programmed cell death mediated by RIPK1, RIPK3 and MLKL downstream of TNFR1 and other receptors and has been implicated in mediating many human diseases. However, whether necroptosis is regulated by phase separation is not yet known. Here, we show that upon the induction of necroptosis and recruitment by the adaptor protein TAX1BP1, PARP5A and its binding partner RNF146 are rapidly induced to form liquid-like condensates by multivalent interactions to perform poly ADP-ribosylation (PARylation) and PARylation-dependent ubiquitination (PARdU) of activated RIPK1. We show that PARdU predominantly occurs on the K376 residue of RIPK1, which promotes proteasomal degradation of kinase-activated RIPK1 to restrain the formation of necrosomes. This modification is essential in vivo, as PARP5A deficiency sensitized mice to TNFα-induced lethal shock in a RIPK1kinase-dependent manner. Our data demonstrate that PARdU on K376 RIPK1 provides an alternative cell death checkpoint mediated by phase separation-dependent control of necroptosis by PARP5A and RNF146.