Autistic adolescents and emerging adults experience high rates of mental health conditions--a risk factor for hospitalization. Using nationally representative data from the 2016 HCUP KID, we estimated the prevalence and costs of mental health-related hospitalizations for autistic adolescents and emerging adults in the United States. Hospital discharges for ages 10-20 years (n = 1,346,849) were analyzed using survey-adjusted logistic regression and general linear modeling. Mental health-related hospitalizations among autistic young people (n = 14,368) were mostly for neurodevelopmental, disruptive, depressive, and bipolar disorders. Mental health-related hospitalizations were almost 11 times more likely (OR = 10.98, 95% CI = 10.00-12.00) for autistic youth compared youth with complex and chronic conditions, and two times (OR = 2.03, 95% CI = 1.88-2.19) more likely compared to population controls. Predicted mean service delivery costs of mental health-related hospitalizations for autistic adolescents and emerging adults were US$7,401.23 per stay, resulting in US$106 million in estimated total service delivery costs in 2016. Mental health-related hospitalizations can indicate poor quality care, be traumatic, and increase suicide risk. Community mental health care for autistic young people is needed.