Earth, Venus, Mars and some extrasolar terrestrial planets(1) have a mass and radius that is consistent with a mass fraction of about 30% metallic core and 70% silicate mantle(2). At the inner frontier of the Solar System, Mercury has a completely different composition, with a mass fraction of about 70% metallic core and 30% silicate mantle(3). Several formation or evolution scenarios are proposed to explain this metal-rich composition, such as a giant impact(4), mantle evaporation(5) or the depletion of silicate at the inner edge of the protoplanetary disk(6). These scenarios are still strongly debated. Here, we report the discovery of a multiple transiting planetary system (K2-229) in which the inner planet has a radius of 1.165 +/- 0.066 Earth radii and a mass of 2.59 +/- 0.43 Earth masses. This Earth-sized planet thus has a core-mass fraction that is compatible with that of Mercury, although it was expected to be similar to that of Earth based on host-star chemistry(7). This larger Mercury analogue either formed with a very peculiar composition or has evolved, for example, by losing part of its mantle. Further characterization of Mercury-like exoplanets such as K2-229 b will help to put the detailed in situ observations of Mercury (with MESSENGER and BepiColombo8) into the global context of the formation and evolution of solar and extrasolar terrestrial planets.