There is an urgent yet insufficiently recognised human-rights and public-health crisis unfolding across Europe: the systematic imprisonment of mentally ill individuals who have already been assessed by courts or psychiatric professionals as requiring treatment in secure psychiatric hospitals rather than confinement in correctional facilities. Evidence indicates that structural failings—including bed shortages, procedural delays, and fragmented legal and administrative frameworks—have produced a situation in which thousands of vulnerable individuals remain in prison in direct contravention of judicial orders, clinical assessments, and international human-rights obligations. This constitutes a largely invisible mental-health scandal, obscured by inconsistent data collection, political sensitivities, and the general invisibility of people in custody.