A hearing aid has an audio-shoe interface, and a hearing-aid system includes a hearing aid and an audio shoe. The hearing aid has a housing, in which an undercut and an abutment are provided. The abutment is arranged opposite to the undercut. Undercut and abutment are embodied such that a retaining lug of an audio shoe can be inserted into the undercut by a rotational movement. The abutment is embodied such that a retaining lug inserted into the undercut can only be removed from the undercut again by a rotational movement in the opposite direction. A lock prevents a rotational movement of an inserted audio shoe in the opposite direction. When the electrical contacts between audio shoe and hearing aid are disposed in the undercut, these too are hidden from view and well protected against external influences.