Gymnotrachelus is an extinct monospecific genus of large selenosteid arthrodire placoderm of the Late Devonian known from the Late Famennian Cleveland Shale of Ohio. The type species Gymnotrachelus hydei was originally reconstructed as physically resembling Selenosteus, with slightly smaller orbits (i.e., having a broad, box-like head). Later specimens led to a reappraisal, and now G. hydei is thought to have a more gar-like or barracuda-like build.