Intense emotional distress and impaired information processing have been implicated in reducing a surrogate decision maker’s ability to formulate informed health care decisions for a critically ill patient. The heightened intensity of negative emotions, mental effort, and impaired judgment is consistent with the manifestation of decision fatigue. The aim of this article is to describe the validity and reliability of the Decision Fatigue Scale (DFS) among surrogate decision makers of the critically ill. A convenience sample of 101 surrogate decision makers were administered the DFS and a battery of psychosocial instruments at two time points. The DFS was specified as a unidimensional measure with adequate internal consistency (Cronbach’s αs = .87, .90) and stability reliability. Discriminant validity was established with measures of emotion regulation, anxiety, and depressive symptoms. The DFS is the first subjective measure of decision fatigue for surrogate decision makers of the critically ill that demonstrates satisfactory psychometric properties.