  We present the first spectropolarimetric observations of a hydrogen-free
superluminous supernova at z=0.1136, namely SN 2015bn. The transient shows
significant polarization at both the observed epochs: one 24 days before
maximum light in the rest-frame, and the subsequent at 27 days after peak
luminosity. Analysis of the Q-U plane suggests the presence of a dominant axis
and no physical departure from the main axis at either epoch. The polarization
spectrum along the dominant axis is characterized by a strong wavelength
dependence and an increase in the signal from the first to the second epoch. We
use a Monte Carlo code to demonstrate that these properties are consistent with
a simple toy model that adopts an axi-symmetric ellipsoidal configuration for
the ejecta. We find that the wavelength dependence of the polarisation is
possibly due to a strong wavelength dependence in the line opacity, while the
higher level of polarisation at the second epoch is a consequence of the
increase in the asphericity of the inner layers of the ejecta or the fact that
the photosphere recedes into less spherical layers. The geometry of the
superluminous supernova results similar to those of stripped-envelope core
collapse SNe connected with GRB, while the overall evolution of the ejecta
shape could be consistent with a central engine.
