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We report the detection of a large-scale magnetic field at the surface of theslowly-rotating fully-convective M dwarf Proxima Centauri. Ten circularpolarization spectra, collected from April to July 2017 with the HARPS-Polspectropolarimeter, exhibit rotationally-modulated Zeeman signatures suggestinga stellar rotation period of $89.8 \pm 4.0$ d. Using Zeeman-Doppler Imaging, weinvert the circular polarization spectra into a surface distribution of thelarge-scale magnetic field. We find that Proxima Cen hosts a large-scalemagnetic field of typical strength 200 G, whose topology is mainly poloidal,and moderately axisymmetric, featuring, in particular, a dipole component of135 G tilted at 51$^{\circ}$ to the rotation axis. The large-scale magneticflux is roughly 3 times smaller than the flux measured from the Zeemanbroadening of unpolarized lines, which suggests that the underlying dynamo isefficient at generating a magnetic field at the largest spatial scales. Ourobservations occur $\sim$1 yr after the maximum of the reported 7 yr-activitycycle of Proxima Cen, which opens the door for the first long-term study of howthe large-scale field evolves with the magnetic cycle in a fully-convectivevery-low-mass star. Finally, we find that Proxima Cen's habitable zone planet,Proxima-b, is likely orbiting outside the Alfv\`en surface, where no directmagnetic star-planet interactions occur.

The large-scale magnetic field of Proxima Centauri near activity maximum