diff --git a/src/api/SIGCM2.Domain/Exceptions/MedioInactivoException.cs b/src/api/SIGCM2.Domain/Exceptions/MedioInactivoException.cs
new file mode 100644
index 0000000..92e40c2
--- /dev/null
+++ b/src/api/SIGCM2.Domain/Exceptions/MedioInactivoException.cs
@@ -0,0 +1,16 @@
+namespace SIGCM2.Domain.Exceptions;
+
+/// <summary>
+/// Thrown when a mutation is attempted on a Seccion whose parent Medio is inactive.
+/// Cascades the freeze from Medio → Seccion (REQ-SEC-006).
+/// </summary>
+public sealed class MedioInactivoException : DomainException
+{
+    public int MedioId { get; }
+
+    public MedioInactivoException(int medioId)
+        : base($"El medio {medioId} está inactivo. No se pueden modificar sus secciones hasta reactivarlo.")
+    {
+        MedioId = medioId;
+    }
+}