  Magnetars are highly magnetized young neutron stars that occasionally produce
enormous bursts and flares of X-rays and gamma-rays. Of the approximately
thirty magnetars currently known in our Galaxy and Magellanic Clouds, five have
exhibited transient radio pulsations. Fast radio bursts (FRBs) are
millisecond-duration bursts of radio waves arriving from cosmological
distances. Some have been seen to repeat. A leading model for repeating FRBs is
that they are extragalactic magnetars, powered by their intense magnetic
fields. However, a challenge to this model has been that FRBs must have radio
luminosities many orders of magnitude larger than those seen from known
Galactic magnetars. Here we report the detection of an extremely intense radio
burst from the Galactic magnetar SGR 1935+2154 using the Canadian Hydrogen
Intensity Mapping Experiment (CHIME) FRB project. The fluence of this
two-component bright radio burst and the estimated distance to SGR 1935+2154
together imply a 400-800 MHz burst energy of $\sim 3 \times 10^{34}$ erg, which
is three orders of magnitude brighter than those of any radio-emitting magnetar
detected thus far. Such a burst coming from a nearby galaxy would be
indistinguishable from a typical FRB. This event thus bridges a large fraction
of the radio energy gap between the population of Galactic magnetars and FRBs,
strongly supporting the notion that magnetars are the origin of at least some
FRBs.
