Tierney made 170 appearances in his previous Celtic spell, winning 11 major honours and will now return to the Scottish champions after leaving at the end of his Arsenal contract. His move was confirmed in February, with reports suggesting that he has fallen out with manager Mikel Arteta who rarely started the defender after taking over the side.
Tierney admitted he is thrilled to be a Celtic player again however, as he spoke to the club website after being announced as one of their first summer signings.
“Everybody’s known for a few months and just to be here with the Celtic strip on again just feels amazing,” he said.
“I spoke with the manager, and obviously I had worked with him before and I’ve always kept in touch with him. He’s one of the best so I’m so lucky and grateful that he’s given me the chance to come back again.
“When you move away from home, you grow up, you learn different things. I was in Spain for a year, learning a different language and playing in a different league, so I’m definitely a different person.
“Player-wise, it’s the same as I always say – I’ll try and work my hardest, I’ll do my best for the club and that’s all I can do.
“My ambition is just for Celtic to be as successful as possible, retain our place at the top and continue to win trophies and do well for Celtic.”
Celtic boss Brendan Rodgers also spoke out after Tierney's return was made official on Tuesday afternoon in what is an emotional reunion for both of them.
"He is such a talented, high-quality player, and he will be a massive addition to our squad.
"Kieran has always had Celtic in his blood, and he is excited to be coming back to the place he loves.
"He is someone who has given so much to Celtic already, and I know he is so motivated and hungry to help the club bring more and more success to our fans.
"We are so pleased to bring such an elite player back to the club, and I look forward to working with him again and the rest of the squad, as we face the many challenges ahead at home and in Europe.”
Former Celtic and Scotland captain Scott Brown also spoke to the BBC about the deal, stating that it’s the perfect move for both parties.
“He's a very, very good character in and about the place. Everyone loves him because whoever he goes to, he gives 110%. You can see how good he's got going down the road, working with probably even better quality players as well, playing in the Champions League, then going away to Spain.
“It's a really good fit for the club and also a really good fit for him as well.”