Believed to be 15, the youth crammed himself into a tiny cavity underneath the coach before clinging on for hours.
The boy looked disheveled as he was pulled from the coach covered in dirt and engine oil.
The coach had travelled from the northern city of Tetouan in Morocco, near Tangier, all the way to Andalucia’s Sevilla, taking several hours.
A Spanish police spokesman said: “He was taken to hospital but appeared to be in good condition.
“He called the police and firefighters, and they found the boy who was holding onto the bottom of the bus.”
He will likely be returned to a migrant holding centre.