Scott, who signed for Athletic from Darlington in the summer, re-joins the Blues to replace outgoing midfielder Jimmy Beadle.
“I’m pleased to get Leon over the line and bring him in to the club,” said Hardy.
“We know what he’s about. He’s very, very experienced, knows the level inside-out and has won honours too and he will add to the group.
“He’s different to a lot of our other midfielders in the group too in that he’ll happily break up play and sit a bit deeper.”
Scott’s signing comes on the back of Hardy bringing back popular midfield man Callum Patton and extending defender Toby Lees’ loan from Harrogate Town for a third month.
Josh Nearney has left Whitby Town this week.