Context. A precise comparison of the predicted and observed locations of stars in the H-R diagram is needed when testing stellar interior theoretical models. For doing this, one must rely on accurate, observed stellar fundamental parameters (mass, radius, luminosity, and abundances). Aims. We determine the angular diameter of the rapidly oscillating Ap star, γ Equ, and derive its fundamental parameters from this value. Methods. We observed γ Equ with the visible spectro-interferometer VEGA installed on the optical CHARA interferometric array, and derived both the uniform-disk angular diameter and the limb-darkened diameter from the calibrated squared visibility. We then determined the luminosity and the effective temperature of the star from the whole energy flux distribution, the parallax, and the angular diameter. Results. We obtained a limb-darkened angular diameter of 0.564  ±  0.017 mas and deduced a radius of R = 2.20 ± 0.12   R⊙. Without considering the multiple nature of the system, we derived a bolometric flux of (3.12 ± 0.21) × 10-7 erg cm-2 s-1 and an effective temperature of 7364  ±  235 K, which is below the previously determined effective temperature. Under the same conditions we found a luminosity of L = 12.8 ± 1.4   L⊙. When the contribution of the closest companion to the bolometric flux is considered, we found that the effective temperature and luminosity of the primary star can reach  ~100 K and  ~0.8 L⊙ lower than the values mentioned above. Conclusions. For the first time, and thanks to the unique capabilities of VEGA, we managed to constrain the angular diameter of a star as small as 0.564 mas with an accuracy of about 3% and to derive its fundamental parameters. In particular the new values of the radius and effective temperature should bring further constraints on the asteroseismic modeling of the star.