  The tidal disruption of a star by a supermassive black hole leads to a
short-lived thermal flare. Despite extensive searches, radio follow-up
observations of known thermal stellar tidal disruption flares (TDFs) have not
yet produced a conclusive detection. We present a detection of variable radio
emission from a thermal TDF, which we interpret as originating from a
newly-launched jet. The multi-wavelength properties of the source present a
natural analogy with accretion state changes of stellar mass black holes,
suggesting all TDFs could be accompanied by a jet. In the rest frame of the
TDF, our radio observations are an order of magnitude more sensitive than
nearly all previous upper limits, explaining how these jets, if common, could
thus far have escaped detection.
