Subsurface horizon assignment. At least some of the illustrative embodiments are methods including: obtaining, by a computer system, a seismic data volume; identifying, by the computer system, a plurality of patches in the seismic data volume, and the identifying thereby creating a patch volume; displaying, on a display device, at least a portion of the seismic data volume and the plurality of patches of the patch volume; and assigning a patch of the plurality of patches to a subsurface horizon of the seismic data volume.