“Multiferroic” materials possessing simultaneously magnetic and ferroelectric orders are scarce and most of them order at low temperatures. So far, bismuth ferrite, BiFeO₃, is the only reliable room-temperature multiferroic: it is ferroelectric and antiferromagnetic. The absence of a net magnetisation in this compound is a problem when one wants to use the magneto-electric effect to address magnetic information with an electric field for potential applications in spintronic devices. We show here that β-NaFeO₂ is also a multiferroic material at room-temperature but with the most interesting extra property of showing weak ferromagnetism. This makes it a potentially very promising material for applications and a model compound for fundamental studies of the interaction between ferroelectricity and magnetism.