The effect of a typical solar energetic particle event on the Martian atmosphere is described. Sputtering, Lyman α emission, atmospheric heating, and the radiation flux at the surface are calculated using a model of the solar wind interaction with the Martian atmosphere. The effect of the crustal magnetic field is also investigated. We provide estimates of the associated escape flux of neutral atoms and molecules. We compare the atmospheric energy deposition due to the incident solar energetic particles with the energy deposition due to other sources such as solar EUV/IR, solar wind protons and Martian energetic neutral atoms derived from pickup ions. We show that atmospheric sputtering efficiency is small compared with that for pickup ions for the present epoch but that the energy deposition at the homopause is important compared with the EUV/IR energy deposition. While some areas of the atmosphere are shielded from the incident flux by the crustal magnetic fields, the energetic particle flux reaching the surface is virtually unattenuated. This study of space weather at Mars raises a number of questions that need to be addressed with new observations and models.