Background: Language assessment is a key element of evaluations of children and adolescents with neurodevelopmental disorders (NDDs). The present study examined the validity of a gaze-based receptive language index (RLI) in predicting language test results.Method: Participants included toddlers, pre-school, and school age children and adolescents (78.1% male; ages 1.8-17.6, M = 6.7, SD = 3.6) referred for evaluation of NDDs. A brief eye-tracking evaluation was completed that included six stimuli with 16 receptive language targets. The RLI was computed by standardising gaze metrics and averaging within and across receptive language targets. Results: The RLI had strong validity for predicting traditional language scores (r = 0.48-0.51) and sensitivity to language impairment (AUC = 0.78). Validity was maintained when controlling for potential confounding factors, including age, autism symptom severity, and eye tracking validity indicators. Conclusions: After large sample validation and scaling for clinical use, the RLI could enhance language evaluation in individuals with NDDs, including autism and intellectual disability.