Soft gamma-ray repeaters are transient sources of high-energy photons; they emit sporadic and short (about 0.1 s) bursts of 'soft' gamma-rays during periods of activity which are often broken by long stretches of quiescence. These objects are associated with neutron stars in young supernova remnants(1). The event of 5 March 1979 was the most intense burst to date, and the only one that showed a dear periodicity in the signal(2,3). Here we report the detection, on 27 August 1998, of an even more intense burst from a different soft gamma-ray repeater. This event was characterized by 'hard' gamma-rays at its peak, followed by a tail 300 s long with a soft spectrum and a dear periodicity of 5.16 s. The burst was probably initiated by a massive disruption of the crust of the neutron star, followed by an outflow of energetic particles rotating with the period of the star. A comparison of the events of 27 August 1998 and 5 March 1979 supports the idea that magnetic energy plays an important role in the genesis of such events. Although these giant hares are rare, they are not unique events and may occur at any time in a neutron star's activity cycle.