The 33-year-old, who has 31 Scotland caps, came through the Hibs youth ranks and won the League Cup with them before joining Rangers in 2007.
Whittaker told the official Hibs site: "I am delighted to be back at Hibernian. When I decided to return to Scotland, Hibernian was the club I hoped to come to.
"While the discussions have taken a while, they have always been positive and I am very pleased to be back playing at Easter Road for the Hibernian fans, and I hope I can play my part in ensuring a successful return to the top flight."
Whittaker, whose last club was Norwich City, will not be available for Saturday's opening League Cup game against Montrose as he requires international clearance.