Receptor-interacting protein kinase 3 (RIPK3) is a vital protein involved in the downstream pathway of necroptosis, a programmed cell death process that helps prevent viral infections and cancers. Activation of necroptosis requires the self-assembly of RIPK3 into amyloid fibrils through its receptor-interacting protein homotypic interaction motif (RHIM), which promotes MLKL phosphorylation and membrane rupture. Our study revealed the existence of a liquid phase of YPet-tagged murine RIPK3-RHIM (YPet-mR3) in vitro. Under low urea conditions, the liquid-liquid phase separation (LLPS) of YPet-mR3 results in the formation of viscous but reversible condensates distinct from the fibrillar assemblies commonly reported. At intermediate urea levels, YPet-mR3 bypasses LLPS and transforms into extended amyloid fibrils. The fibrillization process can be delayed by increasing protein concentrations, adding cations, or introducing molecular crowders, which sequester protein molecules as metastable condensates. At high urea concentrations, the fibrilization of YPet-mR3 is markedly slowed, exceeding 48 hours. In the presence of molecular crowders, YPet-mR3 initially forms reversible condensates, and irreversible fibrils develop from the surface of these condensates in 24 hours. Our findings suggest that condensates are a reversible intermediate in YPet-mR3 self-assembly, acting as a reservoir that hinders the formation of irreversible amyloid fibrils. On the other hand, the surface of condensates can catalyse the fibril formation. These fundamental insights into the dual role of YPet-mR3 phase separation in fibrillization may provide a molecular basis for understanding how RIPK3 assembly modulates necroptotic immune signalling.