Nearly all young stars are initially surrounded by 'protoplanetary' disks of gas and dust, and in the case of single stars at least 30% of these disks go on to form planets(1). The process of protoplanetary disk formation can result in initial misalignments, where the disk orbital plane is different from the stellar equator in single-star systems, or different from the binary orbital plane in systems with two stars(2). A quirk of the dynamics means that initially misaligned 'circumbinary' disks-those that surround two stars-are predicted to evolve to one of two possible stable configurations: one where the disk and binary orbital planes are coplanar and one where they are perpendicular (a 'polar' configuration)(3-5). Previous work has found coplanar circumbinary disks(6), but no polar examples were known until now. Here, we report the first discovery of a protoplanetary circumbinary disk in the polar configuration, supporting the predictions that such disks should exist. The disk shows some characteristics that are similar to disks around single stars, and that are attributed to dust growth. Thus, the first stages of planet formation appear able to proceed in polar circumbinary disks.