Detection of lightning discharges on Venus has been attempted using both radio waves and optical methods for over 40 years. For optical observations, claims of lightning detection were controversial due to the lack of time resolution of optical emissions that is needed to separate lightning from artificial or natural noise. Here we show the first high-time-resolution light curve of a transient optical phenomenon observed by the Lightning and Airglow Camera (LAC), a dedicated instrument on the Venus orbiter Akatsuki. The observed transient was 10 times brighter than a typical terrestrial lightning flash and had a duration of a few hundred milliseconds, whereas that of typical Earth lightning is only a millisecond. These characteristics are not typical, but are well within the variability of Earth lightning. An origin as a bolide flare cannot be excluded, but considering the expected occurrence frequency of meteoroids at Venus, is improbable. The low flash rate and long duration determined by the Akatsuki observation are not inconsistent with non-detection of lightning radio waves by the Cassini spacecraft.