Kieran Tierney has completed his Scottish record £25m move to Arsenal after making what he called "the hardest decision in my whole life".
The 22-year-old left-back completed a medical on Thursday after the clubs agreed terms following lengthy talks.
The English Premier League side previously had two bids rejected by the Scottish Premiership club this summer.
"It's a hard decision to leave a club that you love so much. People need to know that," Tierney said.
Celtic insisted they "did all we could" to keep the Scotland defender, who had come through the club's youth academy.
"While the club did not need or wish to sell Kieran, this was an opportunity Kieran very much wished to pursue and with the club having received a significant offer, we have agreed to the transfer," read a club statement.
Tierney won four league titles with Celtic and has been on the winning side for two Scottish Cups and two League Cups.
However, he has been restricted to just nine appearances this year and had a double hernia operation in the summer.
Celtic thanked Tierney for making a "significant contribution", while manager Neil Lennon reiterated it was "an opportunity Kieran wanted to take".
"Although he is still a young guy, Kieran has given the club so much," Lennon added. "I'm sure he will continue to progress and develop and make a big impact."
'I was down the park with my friends'
Tierney was told at 19:00 BST on Wednesday - just as Celtic kicked off in their Champions League qualifier against Cluj - that he was to travel to London.
"I was actually down the park with my friends training," he told the Arsenal website. "They play with their local team so I was down watching them and I got a phone call.
"I got to the airport and the flight was delayed two-and-a-half hours, with another hour's drive for a hotel and I probably got zero sleep, so I was back up at half five."
Tierney, who has signed a long-term deal and will wear the number three shirt at Arsenal, described the move as a "great opportunity" at a "massive club".
"When I was younger, the ambition was Celtic and that was my dream," he added. "But when you get older... 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."
Unai Emery, his new coach, said he was "delighted" to have completed the deal for "a very talented player".
'This could be very good for him' - analysis
BBC Scotland chief sports writer Tom English
It's the right age and the right time for Celtic and Kieran Tierney. He's won everything he can at Celtic outside of Europe and he's going to Arsenal to try and do something in Europe, the Premier League, advance his career, stake his claim for that Scotland left-back spot.
He just needs to get fit and try to put his best foot forward. This could be very good for him.
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