Cox said: "It was a quick move - the deal was done in about half an hour.
"The manager [Dave Jones] has brought me in for my experience. I have played over 500 games so hopefully the experience of the more senior players can help the youngsters in the side."
Cox joined the Iron as a schoolboy in 1990 and made 26 appearances before joining Aston Villa for £350,000 in February, 1991.
The former England-U21 has had spells at Bolton and Middlesbrough, but has always said that he would love to return to Glanford Park at some stage in his career.