Purpose: This study examined sleep problems in Flemish (Dutch speaking part of Belgium) school-aged children (4–12 years) placed in family foster care. Research design: Foster parents (n = 197) completed an online questionnaire consisting of: (1) characteristics of foster child, foster family and foster placement, (2) behavioural problems of foster child, and (3) sleep problems of foster child using the Dutch version of the ‘Children’s Sleep Habits Questionnaire (CSHQ)’. Higher scores indicate more sleep problems. Results: Based on a cut-off value used in an American sample, 71.1% of the foster children in our sample screened positive for clinically significant sleep problems. In general, foster children scored significantly higher than children from different community samples and significantly lower than children from different sleep-disordered samples. Multiple regression analysis showed that sleep problems were significantly associated with internalizing problem behaviour, externalizing problem behaviour, and household composition. The likelihood of experiencing/perceiving sleep problems in foster children was higher in case of more internalizing or externalizing problem behaviour and in case of single parenthood. Conclusion: These results highlight the importance of screening, assessment and intervention in foster children with sleep problems by health professionals.