Aims: We aim at detecting and characterizing the main-sequence companion of the Cepheid AX Cir ($P_mathrm{orb} sim $ 18 yrs). The long-term objective is to estimate the mass of both components and the distance to the system. Methods: We used the PIONIER combiner at the VLT Interferometer to obtain the first interferometric measurements of the short-period Cepheid AX Cir and its orbiting component. Results: The companion is resolved by PIONIER at a projected separation $rho = 29.2 pm 0.2$ mas and projection angle $PA = 167.6 pm 0.3^{circ}$. We measured $H$-band flux ratios between the companion and the Cepheid of $0.90 pm 0.10$ % and $0.75 pm 0.17$ %, respectively at a pulsation phase for the Cepheid $phi = 0.24$ and 0.48. The lower contrast at $phi = 0.48$ is due to increased brightness of the Cepheid compared to the $phi = 0.24$. This gives an average apparent magnitude $mmathrm{_H (comp)} = 9.06 pm 0.24$ mag. The limb-darkened angular diameter of the Cepheid at the two pulsation phases was measured to be $theta_mathrm{LD} = 0.839 pm 0.023$ mas and $theta_mathrm{LD} = 0.742 pm 0.020$ mas, respectively at $phi = 0.24$ and 0.48. A lower limit on the total mass of the system was also derived based on our measured separation, we found $M_mathrm{T} geq 9.7 pm 0.6 M_odot$.
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