The 33-year-old has been struggling with injuries at Ibrox in recent seasons.
He told the club’s official website: “It is a difficult moment for me to leave Rangers, because I played for a year-and-a-half here and I was very happy in Glasgow.
“Last year was my best season because we won two cups and I scored lots of goals.
“It was a disappointment to lose the SPL in the last game and lose the UEFA Cup final, but I can’t forget what happened, as there were still a lot of good memories.
“It has been a special time. I have good friends here who I can call when I leave and I’ll always be a Rangers fan.”