The aim of this study was to assess test-retest reliability and diagnostic validity of self-report instruments of depression and anxiety in autistic youth. Participants were 55 autistic youth aged 8-17 years presenting with depressive or anxiety symptoms. They were interviewed with the Kiddie Schedule for Affective Disorders and Schizophrenia for School-Age Children (K-SADS-PL) and completed the Children's Depression Inventory, Second Edition -- Self Report Short (CDI 2:SR[S]) and the Revised Child Anxiety and Depression Scale (RCADS) twice, separated by a two-week interval. Test-retest reliability was measured with intraclass correlation coefficients (ICCs), and diagnostic validity was assessed using receiver operating characteristic (ROC) curves with the summary ratings on the K-SADS-PL as the criterion. The effect of participant characteristics was analyzed through a moderation analysis. Generalized anxiety (GAD) and social anxiety disorder (SOC) were the two most prevalent disorders in the sample. Test-retest reliability for most of the subscales was good (ICC = 0.74 - 0.87), with the exception of the RCADS obsessive-compulsive disorder (OCD) and GAD. The Adaptive Behavior conceptual score was a significant moderator of the reliability of the CDI 2:SR[S]. The ROC analysis suggested the RCADS SOC and the CDI 2:SR[S] to be good screening tools with inadequate specificity when appropriately sensitive cutoff scores are used. Optimal cutoff scores in this sample were lower than originally published. The findings suggest that autistic youth can provide stable reports of anxiety and depressive symptoms over time. Diagnostic validity varied according to the construct and instrument.