  We present late-time radio observations of GW170817, the first binary neutron
star merger discovered through gravitational waves by the advanced LIGO and
Virgo detectors. Our observations, carried out with the Karl G. Jansky Very
Large Array, were optimized to detect polarized radio emission, and thus to
constrain the linear polarization fraction of GW170817. At an epoch of ~244
days after the merger, we rule out linearly polarized emission above a fraction
of ~12% at a frequency of 2.8 GHz (99% confidence). Within the structured jet
scenario (a.k.a. successful jet plus cocoon system) for GW170817, the derived
upper-limit on the radio continuum linear polarization fraction strongly
constrains the magnetic field configuration in the shocked ejecta. We show that
our results for GW170817 are compatible with the low level of linear
polarization found in afterglows of cosmological long gamma-ray bursts.
Finally, we discuss our findings in the context of future expectations for the
study of radio counterparts of binary neutron star mergers identified by
ground-based gravitational-wave detectors.
