Parental conflict regulates seed development, with maternal and paternal genomes respectively suppressing and promoting nutrient supply to the embryo. The maternally expressed Polycomb Repressive Complex 2 (PRC2) inhibits endosperm development, yet the corresponding antagonistic paternal factor is not well understood. Mutations in the PRC2 subunit gene MEDEA result in a collapsed-seed phenotype. In this study, we identified a paternally expressed imprinted gene (PEG) named ‘AEGEUS’ (AGU), after the Greek mythological king who married Medea, known for her fertility. AGU encodes a RING finger protein, and paternal mutations lead to early endosperm cellularization. AGU serves as a central node in the paternal gene network, which includes PEGs and non-imprinted transcription factors, notably the endosperm cellularization gene AGL62. Our findings suggest that MEDEA and AEGEUS have opposing roles in regulating the timing of endosperm cellularization and, consequently, seed size by harmoniously controlling the downstream network in seed development.