Background. Children with a brain tumor are at risk of developing sleep problems. It remains unclear whether these problems arise at an early or later stage, and insights can facilitate timely interventions. The aim of this study is to examine sleep problems and contributing factors shortly after diagnosis. Methods. Children 6-16 years with a newly diagnosed (≤3 months) primary brain tumor were recruited for a prospective study. Sleep was measured using actigraphy and questionnaires (PROMIS Sleep Disturbance and Sleep Related Impairment, self- and parent-reports). Prevalence of clinical sleep problems were established using PROMIS cut-off scores. Mean PROMIS scores, prevalence of sleep problems and actigraphic outcomes were compared to norms (t-test, chi-square, linear regression). Demographic and medical risk factors were explored with multivariable linear regression models. Results. Sixty-nine children (68% male, mean age 11.6±2.8 years, 53±28 days after diagnosis) participated. Parents reported more sleep disturbances (mean T=53.7, P<.01) compared to norms. Rates of self- and parent-reported severe sleep disturbances were elevated (11% versus 5% in norms, P<.04). Parents also reported higher rates of moderate sleep disturbance (31%) and sleep related impairment (42%) than norms (25%, P<.03). Actigraphic outcomes did not differ from norms. Only shorter time since diagnosis was identified as independent risk factor (self-reported sleep disturbances, B=-.11, 95%CI -0.19;-0.03). Conclusions. Sleep problems are more frequently reported by children and parents shortly after pediatric brain tumor diagnosis, compared to healthy controls. Attention for sleep around brain tumor diagnosis is important, as sleep is vital for recovery and health-related quality of life.