ABSTRACT
Magnetic fields are fundamental to the evolution of galaxies, playing a key role in the astrophysics of the interstellar medium and star formation. Large-scale ordered magnetic fields have been mapped in the Milky Way and nearby galaxies1,2, but it is not known how early in the Universe such structures formed3. Here we report the detection of linearly polarized thermal emission from dust grains in a strongly lensed, intrinsically luminous galaxy that is forming stars at a rate more than 1,000 times that of the Milky Way at redshift 2.6, within 2.5 Gyr of the Big Bang4,5. The polarized emission arises from the alignment of dust grains with the local magnetic field6,7. The median polarization fraction is of the order of 1%, similar to nearby spiral galaxies8. Our observations support the presence of a 5-kiloparsec-scale ordered magnetic field with a strength of around 500 µG or lower, oriented parallel to the molecular gas disk. This confirms that such structures can be rapidly formed in galaxies, early in cosmic history.