Electronic phases with symmetry properties matching those of conventional liquid crystals have recently been discovered in transport experiments on semiconductor heterostructures and metal oxides at milli-Kelvin temperatures. We report the spontaneous onset of a onedimensional, incommensurate modulation of the spin system in the high-temperature superconductor YBa 2 Cu 3 O 6.45 upon cooling below ~150 K, while static magnetic order is absent above 2 K. The evolution of this modulation with temperature and doping parallels that of the in-plane anisotropy of the resistivity, indicating an electronic nematic phase that is stable over a wide temperature range. The results suggest that soft spin fluctuations are a microscopic route towards electronic liquid crystals, and nematic order can coexist with high-temperature superconductivity in underdoped cuprates.The electronic states near the Fermi level of high-temperature superconductors derive from the hybridized d-and p-orbitals of copper and oxygen ions in a square-planar network. At a doping level of 1/8 hole per Cu ion, experimental work on a specific superconducting cuprate family, (La,Nd) 2-x (Sr,Ba) x CuO 4 (La214), has shown that the two-dimensional electron system in the CuO 2 layers can support a state with uniaxial spin (1-3) and charge (1,4,5) order ("stripes"). Static stripe order implies that both translational and rotational symmetries of the copper-oxide square lattice are spontaneously broken. More unusual "electronic liquid crystal" states (6) that break the rotational symmetry of the lattice while at least partially preserving its translational symmetry can arise from quantum fluctuations of stripes(6-8), or from Fermi surface instabilities (9-11). Electronic nematic states have recently been discovered in semiconductor heterostructures (12) and in the bulk transition metal oxide Sr 3 Ru 2 O 7 (13). In both cases, however, they are stable only at milli-Kelvin temperatures and in high magnetic fields, and have thus far only been probed by transport measurements. We use neutron scattering to address the role of magnetic degrees of freedom in driving the formation of electronic liquid crystals, and to explore the presence of liquid-crystalline order in the cuprates.In the prior experiments on semiconductor heterostructures and on Sr 3 Ru 2 O 7 , the nematic director was aligned by external magnetic fields, resulting in a strong macroscopic anisotropy of 2 the current flow (12,13), analogous to the alignment of nematic domains in conventional liquid crystals by electric fields or confining walls. In the cuprates, subtle crystallographic distortions can serve as aligning fields for symmetry-broken electronic phases. They reduce the fourfold rotational symmetry of the CuO 2 layer to a twofold rotational or mirror symmetry by introducing a slight (~1%) difference between the in-plane lattice parameters. In stripe-ordered La214, the stripe domains in every CuO 2 layer are aligned by such a twofold axis, but the layers are stacked in such a way that the glob...