“Everybody’s known for a few months and just to be here with the Celtic strip on again just feels amazing.
“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.”
Tierney had gone on loan to Sociedad after falling out of favour under Mikel Arteta amid injury problems and failed to become a regular starter on his return, and all but four of his 20 appearances this season were as a substitute.
While in the final few months of his contract at Arsenal, Tierney’s return to Celtic Park had been confirmed in February.
This season, Brendan Rodgers’ side won the Scottish Premiership and League Cup, while also reaching the knockout stage of the Champions League.
The Celtic boss said: “He is such a talented, high quality player and he will be a massive addition to our squad.
“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.”
Tierney arrives amid uncertainty about the future of fellow Scotland left-back Greg Taylor, whose contract is ending this summer.
