Establishing the nature of gamma-ray bursts is one of the greatest challenges in high-energy astrophysics. The distribution of these bursts is isotropic across the sky, but inhomogeneous in space, with a deficit of faint bursts(1). It is currently unknown whether gamma-ray bursts are produced in our Galaxy or at cosmological distances. The detection and identification of counterparts at other wavelengths are seen as crucial for resolving the origin of the events. Here we report the detection by the Beppo-SAX satellite(2) of an X-ray ’afterglow’, associated with the gamma-ray burst of 28 February 1997 (GRB970228; ref. 3)-the first such detection for any gamma-ray burst. The X-ray transient was found to contain a significant fraction of the total energy of the gamma-ray burst and, following the initial detection(4) eight hours after the main burst, faded within a few days with a power-law decay function. The rapid locating of this gamma-ray burst instigated a multi-wavelength observational campaign that culminated in the identification(5) of a fading optical transient in a position consistent(6) with the X-ray transient reported here.