We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of the F9 V star β Virginis (β Vir). After correcting for limb darkening and combining with the revised Hipparcos parallax, we derive a radius of 1.703 ± 0.022 R⊙ (1.3 per cent). We have also calculated the bolometric flux from published measurements which, combined with the angular diameter, implies an effective temperature of 6059 ± 49 K (0.8 per cent). We also derived the luminosity of β Vir to be L = 3.51 ± 0.08 L⊙ (2.1 per cent). Solar-like oscillations were measured in this star by Carrier et al. and using their value for the large frequency separation yields the mean stellar density with an uncertainty of about 2 per cent. Our constraints on the fundamental parameters of β Vir will be important to test the theoretical models of this star and its oscillations.