SummarySeed size in higher plants is coordinately determined by the growth of the embryo, endosperm and maternal tissue, but relatively little is known about the genetic and molecular mechanisms that set final seed size. We have previously demonstrated that Arabidopsis DA1 acts maternally to control seed size, with the da1‐1 mutant producing larger seeds than the wild type. Through an activation tagging screen for modifiers of da1‐1, we have identified an enhancer of da1‐1 (eod3‐1D) in seed size. EOD3 encodes the Arabidopsis cytochrome P450/CYP78A6 and is expressed in most plant organs. Overexpression of EOD3 dramatically increases the seed size of wild‐type plants, whereas eod3‐ko loss‐of‐function mutants form small seeds. The disruption of CYP78A9, the most closely related family member, synergistically enhances the seed size phenotype of eod3‐ko mutants, indicating that EOD3 functions redundantly with CYP78A9 to affect seed growth. Reciprocal cross experiments show that EOD3 acts maternally to promote seed growth. eod3‐ko cyp78a9‐ko double mutants have smaller cells in the maternal integuments of developing seeds, whereas eod3‐1D forms more and larger cells in the integuments. Genetic analyses suggest that EOD3 functions independently of maternal factors DA1 and TTG2 to influence seed growth. Collectively, our findings identify EOD3 as a factor of seed size control, and give insight into how plants control their seed size.