Kieran Tierney has signed for Arsenal from Celtic on Deadline Day for a fee of £25m.
Arsenal agreed a fee with Celtic for the Scotland international left-back on Wednesday and have paid a record fee for a Scottish player.
Their initial £15m bid was rejected in June, as was a second £25m offer as it contained too many add-ons and was dependent on Arsenal qualifying for the Champions League, but their third was finally accepted.
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The 22-year-old has been one of the Gunners' top transfer targets all summer and will wear the No 3 shirt at the Emirates.
Tierney told the club's official website: "It feels great. I'm delighted to be here. Obviously I'm injured now, but I'm just looking forward to getting back on the pitch, training and getting involved with the squad.
"When I was younger, [my ambition] was always Celtic. The ambition was Celtic and that was my dream when I was younger.
"I've done that, I've made my dream, I've lived my dream and I've loved every single minute of it. Now I just feel was the right time to take a step on. I feel this is a great opportunity for me."
Tierney won five Scottish Premiership titles and made 40 appearances in all competitions for Celtic during the 2018/19 campaign.
Arsenal head coach Unai Emery added: "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."
He becomes Emery's fifth signing of the summer window - joining Nicolas Pepe (Lyon), William Saliba (Saint-Etienne), Gabriel Martinelli (Ituano) and Dani Ceballos (Real Madrid, loan).
Sky Sports' Charlie Nicholas:
"As a Celtic and Arsenal fan, I'm happy Arsenal have identified they need a stronger left back. Tierney has improved a lot, he'd be better than what Arsenal currently have. I think it's a high price, he's been a treble-treble winner and it's been an awkward situation for him, but I think the time is right for him to move on.
"Celtic fans might ask why he doesn't stay for nine in a row. That's not guaranteed, neither is 10. Going to a big club like Arsenal, in the Premier League, you can't guarantee trophies, but they are signing a very good left back who can get better. And I think they needed to make a statement of intent defensively because they look very exciting going forward."