People who lived in foster or residential care have a higher risk of suicidal ideation and behaviour. However, most previous research has neglected applying suicide theories to help understand this risk. This paper presents findings from a cross-national study using the Interpersonal-Psychological Theory of Suicide (IPTS) to explore why care-experienced people, especially those leaving care, are more likely to have suicidal thoughts. The findings show associations between the IPTS factors, perceived burdensomeness and thwarted belongingness, and suicidal ideation among care-experienced young adults from England and Germany. This study contributes to helping develop necessary suicide prevention strategies across national care systems.