A mouth guard that can be customized in the field without needing to boil the mouth guard is disclosed. The mouth guard comprises a facial and lingual walls mounted on a thin flexible tray, such as a fabric mesh. Stops are positioned on the tray to space the user's teeth away from the tray. A layer of uncured elastomeric impression material is applied to the tray and the user bites down on the mouth guard to custom fit the mouth guard to the shape of the user's upper and lower teeth. The facial wall can be made of separate spaced-apart walls that can also be filled with impression or impact absorbing material.