  GRB 130925A is one of the recent additions to the growing family of
ultra-long GRBs (T90$ \gtrsim 1000$ s). While the X-ray emission of ultra-long
GRBs have been studied extensively in the past, no comprehensive radio dataset
has been obtained so far. We report here the early discovery of an unusual
radio afterglow associated with the ultra-long GRB 130925A. The radio emission
peaks at low-frequencies ($\sim 7$ GHz) at early times, only $2.2$ days after
the burst occurred. More notably, the radio spectrum at frequencies above $10$
GHz exhibits a rather steep cut-off, compared to other long GRB radio
afterglows. This cut-off can be explained if the emitting electrons are either
mono-energetic or originate from a rather steep, $dN/dE \propto E^{-4}$,
power-law energy distribution. An alternative electron acceleration mechanism
may be required to produce such an electron energy distribution. Furthermore,
the radio spectrum exhibits a secondary underlying and slowly varying
component. This may hint that the radio emission we observed is comprised of
emission from both a reverse and a forward shock. We discuss our results in
comparison with previous works that studied the unusual X-ray spectrum of this
event and discuss the implications of our findings on progenitor scenarios.
