Kieran Tierney will wear the number three shirt at Arsenal, the club have announced.
The left-back was signed from Celtic ten minutes before the transfer window slammed shut on Thursday.
Arsenal have been in hot pursuit of the defender all summer and had two bids rejected by Celtic.
They eventually agreed on a fee with the Scottish club on Wednesday, and Tierney drove down to London on the same night.
He has put pen to paper on a long-term deal with the Emirates club.
Arsenal boss Unai Emery said: "We're delighted Kieran is joining us. He's a very talented player who will continue to improve.
"He increases our options defensively and I look forward to him joining our group."
Tierney added: "It feels great. It was the hardest decision in my life to leave a club that you love so much. People need to know that as well.
"I feel Arsenal is a massive club and it's a great opportunity for me as well. I'm delighted to be here."
