The Scotland international recently reached 100 goals for the club with a strike against Lithuanian side FK Suduva and signed the four-year contract after extensive talks with the club.
“I’m over the moon,” he told Celtic TV. “It’s been a long time coming, talks have been ongoing for a while and I’m glad to get it over the line.
“The main aim is to be here for as long as I can, and for Celtic to have shown that faith in me to sign me for another four years makes me ecstatic.
“From the moment I walked in the door here, this is where I wanted to be for the rest of my life. I’ve been here for three-and-a-half years already and to sign another four-year deal is great. I can’t wait for the future.
“Celtic is a great club and I wouldn’t want to be anywhere else. I had the opportunity to move but I wanted to stay and fight for my place. I’ve been rewarded with another deal and knowing that I’m going to be here for another four years is great.”
The 28-year-old has now set his sights on further cementing his place in Celtic folklore by helping the team to win ten successive league titles.
He said: “I’m here for the 10-in-a-row bid, but my main focus now is trying to get in the team and stay as fit as possible. I’ve hit the 100 now and I want to try and continue that and aim for greater heights.
“There’s been a lot of high and some lows, but more highs than lows. Hopefully there’s more highs to come.
“The Invincible season and the Double Treble season were some very good highlights and hopefully in the coming years there’s more to come.”