The merger of two dense stellar remnants including at least one neutron star is predicted to produce gravitational waves (GWs) and short-duration gamma ray burststs. In the process, neutron-rich material is ejected from the system and heavy elements are synthesized by r-process nucleosynthesis(1,3). The radioactive decay of these heavy elements produces additional transient radiation termed kilonova or macronova(4-10). We report the detection of linear optical polarization, P= (0.50 +/- 0.07)%, 1.46 days after detection of the GWs from GW170817-a double neutron star merger associated with an optical macronova counterpart and a short gamma ray burst(11-14). The optical emission from a macronova is expected to be characterized by a blue, rapidly decaying component and a red, more slowly evolving component due to material rich in heavy elements-the lanthanidess(15). The polarization measurement was made when the macronova was still in its blue phase, during which there was an important contribution from a lanthanide-free outflow. The low degree of polarization is consistent with intrinsically unpolarized emission scattered by galactic dust, suggesting a symmetric geometry of the emitting region and low inclination of the merger system. Stringent upper limits to the polarization degree from 2.45-9.48 days post-burst are consistent with the lanthanides-rich macronova interpretation.