We propose and investigate computationally Ag-Au subnanometer clusters as catalysts for the hydrogen evolution reaction (HER). Focusing on Ag12Au, we conduct a complete first-principles study of the HER process on this Au@Ag ultrananocatalyst. After determining the hydrogen-saturated resting state under standard conditions as Ag12AuH11, HER reaction energies and barriers are predicted also including solvent effects using both implicit and explicit models. We find that Ag12Au is a good candidate as a HER catalyst, with good stability and an overall reaction energy barrier of 0.89 eV as an upper bound. We also draw indications for the design of HER subnanometer catalysts.