In Arabidopsis, rapid expansion of the coenocytic endosperm after fertilization has been proposed to drive early seed growth, which is in turn constrained by the seed coat. This hypothesis implies physical heterogeneity between the endosperm and seed coat compartments during early seed development, which to date has not been demonstrated. Here we combine tissue indentation with modelling to show that the physical properties of the developing seed are consistent with the hypothesis that elevated endosperm-derived turgor pressure drives early seed expansion. We provide evidence that whole-seed turgor is generated by the endosperm at early developmental stages. Furthermore, we show that endosperm cellularization and seed growth arrest are associated with a drop in endosperm turgor pressure. Finally we demonstrated that this decrease is perturbed when the function of POLYCOMB REPRESSIVE COMPLEX2 is lost, suggesting that turgor pressure changes could be a target of genomic imprinting. Our results indicate a developmental role for changes in endosperm turgor-pressure in the Arabidopsis seed.