  We report the results of XMM-Newton observations for an especially luminous
supersoft X-ray source (SSS) with bolometric luminosity of 10^39 erg/s in the
spiral galaxy NGC 300. The source was detected as a SSS in 1992 and disappeared
in subsequent X-ray observations. The source was active again during recent
XMM-Newton observations. It appeared to be very soft (kT~60 eV) and very
luminous (~10^38 - 10^39 erg/s). The two XMM-Newton observations also reveal
that the source went from a ``high'' state to a ``low'' state in 6 days. We
also found a 5.4-hr periodicity during the ``low'' state. We consider white
dwarf, black hole, and neutron star models to explain the nature of the source.
