Gamma-ray bursts have been phenomenologically classified into long and short populations based on whether the observed duration is longer or shorter than two seconds. Multi-wavelength and multi-messenger observations in recent years have revealed that in general long GRBs originate from massive star core collapse events, whereas short GRBs originate from binary neutron star mergers. It has been known that the duration criterion is sometimes unreliable, and multi-wavelength criteria are needed to identify the physical origin of a particular GRB. Some apparently long GRBs have been suggested to have a neutron star merger origin, whereas some apparently short GRBs have been attributed to genuinely long GRBs whose short, bright emission is above the detector's sensitivity threshold. Still, there has been no known case that a GRB is genuinely short but originates from death of a massive star. Here we report the comprehensive analysis of the multi-wavelength data of a bright short GRB 200826A. This burst has a sharp 1-second spike, which is not part of an underlying long-duration event. Its other observational properties are, however, inconsistent with those of other short GRBs believed to originate from binary neutron star mergers. Rather, these properties resemble those of long GRBs. This burst presents a challenge to the traditional GRB classification scheme and reveals a class of core-collapse-origin GRBs with genuinly short durations.