Maciej Soltys, 21, was released from ‘a prison in Doncaster’ on Monday, October 11, but hasn't made contact with his family since.
His family originally understood he had been released from Armley Prison in Leeds but they now understand he may have been freed from Doncaster, Moorland or Lindholme jails.
His sister Agata, 25, who lives in Leeds, said: "Nobody has seen him from last Monday. One week is definitely too long.
"Maciej is just a young lad who lost his way. He doesn't do anything horrible. How can we feel in this kind of situation? We are terrified.
"We want to find him and keep helping him to sort his life out. He is and he will always be a part of our family."
Agata said she has informed police about her brother’s disappearance.