A challenging aspect of geriatric practice is that it often requires decision-making under conditions of uncertainty. The Script Concordance Test (SCT) is an assessment tool designed to measure clinical data interpretation, an important element of clinical reasoning under uncertainty. The purpose of this study was to develop and analyze the validity of results of an SCT administered to undergraduate students in geriatric medicine. An SCT consisting of 13 cases and 104 items covering a spectrum of common geriatric problems was designed and administered to 41 undergraduate medical students at a medical school in São Paulo, Brazil. A reference panel of 21 practicing geriatricians contributed to the test's score key. The responses were analyzed, and the psychometric properties of the tool were investigated. The test's internal consistency and discriminative capacity to distinguish students from experienced geriatricians supported construct validity. The Cronbach alpha for the test was 0.84, and mean scores for the experts were found to be significantly higher than those of the students (80.0 and 70.7, respectively; P < .001). This study demonstrated robust evidence of reliability and validity of an SCT developed for use in geriatric medicine for assessing clinical reasoning skills under conditions of uncertainty in undergraduate medical students. These findings will be of interest to those involved in assessing clinical competence in geriatrics and will have important potential application in medical school examinations.