Subjective cognitive function is an important outcome measure in oncology. The Functional Assessment of Cancer Therapy-Brain (FACT-Br) is a quality of life (QoL) measure that includes indices of physical, emotional, social, and neurologic aspects of disease but does not measure cognitive function. This study seeks to validate a novel index of cognition derived from the FACT-Br. Patients with heterogeneous cancer diagnoses (N = 214) completed neuropsychological evaluation and self-report measures. Nine FACT-Br items regarding cognition were combined to form the FACT-Br-cognitive index (CI). Reliability was evaluated by exploratory factor analysis and internal consistency. Concurrent validity was assessed by correlating FACT-Br-CI with the Patient-Reported Outcomes Measurement Information System (PROMIS) Cognitive-8 scales. Discriminant validity was assessed by correlating FACT-Br-CI with other FACT-Br indices and the Beck Depression and Anxiety Inventories (BDI, BAI). Exploratory analyses evaluated the impact of cognitive performance and disease variables on FACT-Br-CI. The FACT-Br-CI consisted of a single factor that demonstrated high internal consistency (α = 0.867) and strong concurrent validity, correlating strongly with PROMIS Cognitive-8 scales (r = 0.675-0.782). The relationship between the FACT-Br-CI and other FACT subscales ranged from moderate to strong (r = 0.372-0.601), as did correlations with measures of depression (BDI, r = -0.621) and anxiety (BAI, r = -0.450). Modest correlations were observed with neuropsychological measures (rs = 0.249-0.300). The FACT-Br-CI is a reliable and valid measure of self-reported cognition. Studies that included the FACT-Br could be retrospectively analyzed to assess subjective cognitive outcomes, enriching the information from prior research. Integration of the FACT-Br-CI in routine clinical care may be an efficient method of monitoring cognition.