The prevalence of posttraumatic stress disorder (PTSD) in very young children depends on the diagnostic criteria. Thus far, studies have investigated the International Classification of Diseases (11th rev.; ICD-11) criteria for PTSD only in samples of children older than 6 years of age. The aim of this study was to test the diagnostic agreement between the ICD-11 and the Diagnostic and Statistical Manual of Mental Disorders (5th ed.; DSM-5) criteria for children who are 6 years old and younger. Caregivers of children aged 3-6 years in foster care in Germany (N = 147) and parents of children aged 1-4 years who had attended a hospital in Switzerland following burn injuries (N = 149) completed a questionnaire about children's PTSD. Rates of PTSD were calculated according to ICD-11 (considering a specific and a more general conceptualization of intrusive memories) and DSM-5 criteria and were compared using McNemar's tests and Cohen's kappa. The proportion of children who met the ICD-11 criteria was 0.6-25.8% lower than the proportion of PTSD cases according to the DSM-5 criteria. The diagnostic agreement between each ICD-11 algorithm and DSM-5 was moderate, κ = 0.52-0.66. A systematic investigation of adaptions of the ICD-11 avoidance cluster identified alternative symptom combinations leading to higher agreement with the DSM-5 requirements. Furthermore, DSM-5 had higher predictive power for functional impairment than the ICD-11 algorithms. In conclusion, the findings suggest that the ICD-11 criteria show less sensitivity in very young children, which can be explained by the more stringent avoidance cluster.