The small and active Saturnian moon Enceladus is one of the primary targets of the Cassini mission. We determined the quadrupole gravity field of Enceladus and its hemispherical asymmetry using Doppler data from three spacecraft flybys. Our results indicate the presence of a negative mass anomaly in the south-polar region, largely compensated by a positive subsurface anomaly compatible with the presence of a regional subsurface sea at depths of 30 to 40 kilometers and extending up to south latitudes of about 50 degrees. The estimated values for the largest quadrupole harmonic coefficients (10(6)J(2) = 5435.2 +/- 34.9, 10(6)C(22) = 1549.8 +/- 15.6, 1 sigma) and their ratio (J(2)/C-22 = 3.51 +/- 0.05) indicate that the body deviates mildly from hydrostatic equilibrium. The moment of inertia is around 0.335MR(2), where M is the mass and R is the radius, suggesting a differentiated body with a low-density core.