With its imaged debris disk of dust, its evaporating exocomets, and an imaged giant planet, the young ( 23 Myr) β Pictoris system is a unique proxy for detailed studies of planet formation processes as well as planet-disk interactions. Here, we study ten years of European Southern Observatory/High Accuracy Radial Velocity Planet Searcher (HARPS) high-resolution spectroscopic data of β Pictoris. After removing the radial velocity (RV) signals arising from the δ Scuti pulsations of the star, a 1,200-d periodic signal remains, which, within our current knowledge, we can only attribute to a second planet in the system. The β Pic c mass is about nine times the mass of Jupiter; it orbits at 2.7 uc(au) on an eccentric (e 0.24) orbit. More RV data are needed to obtain more precise estimates of the properties of β Pic c. The current modelling of the planet's properties and the dynamic of the whole system has to be reinvestigated in light of this detection.