The Magnificent Seven (M7) are a group of radio-quiet Isolated Neutron Stars (INSs) discovered in the soft X-rays through their purely thermal surface emission. Owing to the large inferred magnetic fields ($Bapprox 10^{13}$ G), radiation from these sources is expected to be substantially polarised, independently on the mechanism actually responsible for the thermal emission. A large observed polarisation degree is, however, expected only if quantum-electrodynamics (QED) polarisation effects are present in the magnetised vacuum around the star. The detection of a strongly linearly polarised signal would therefore provide the first observational evidence of QED effects in the strong-field regime. While polarisation measurements in the soft X-rays are not feasible yet, optical polarisation measurements are within reach also for quite faint targets, like the M7 which have optical counterparts with magnitudes $approx 26$--$28$. Here, we report on the measurement of optical linear polarisation for the prototype, and brightest member, of the class, RX, J1856.5$-$3754 ($Vsim 25.5$), the first ever for one of the M7, obtained with the Very Large Telescope. We measured a polarisation degree $mathrm{P.D.} =16.43% pm5.26%$ and a polarisation position angle $mathrm{P.A.}=145fdg39pm9fdg44$, computed east of the North Celestial Meridian. The $mathrm{P.D.}$ that we derive is large enough to support the presence of vacuum birefringence, as predicted by QED.
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