Fuel cells face challenges related to performance and cost that stem in part from constraints of the conventional plate-and-frame architecture in vogue since Volta's time. Here, we present for the first time a cylindrical Swiss-roll mixed-reactant fuel cell (SR-MRFC) that addresses some of these challenges by eliminating ion exchange membranes and flow-field plates, to provide a light, compact multi-cell stack. We demonstrate the monopolar and bipolar SR-MRFC for the borohydride-oxygen system, using a liquid/gas mixture of 1 M NaBH4/2 M NaOH and O2. A 3D anode with Pt/C (or PtRu/C) on carbon cloth was coupled with a gas-diffusion Ag/C (or MnO2/C) cathode. At 323 K and atmospheric pressure, a peak power density of density of 1000 W m−2 was achieved with good performance stability. The reactor concept presented here is applicable to a variety of alkaline or acid fuel cell systems.