Walcott's move has finally gone through
The highly-rated 16-year-old is moving to Arsenal for a fee that is expected to be worth an initial £5m, but rising to about £12.5m.
Walcott said: "I am so pleased to be joining Arsenal - a club I have admired for a long time.
"Coming to Arsenal will give me the opportunity to work with world-class players every day and play football at the highest possible level."
Walcott was unveiled alongside Arsenal manager Arsene Wenger at Arsenal's training ground in London Colney, Hertfordshire.
The striker added: "It is a strange day because I am also very sad to be moving from Southampton.
"I am leaving many friends and some great coaches who have been so important to my career so far, but I leave with many great memories from the club.
"I am now really looking forward to meeting my new team-mates and working hard as an Arsenal player."
Walcott is the most expensive 16-year-old in the history of British football.
Jermaine Pennant was 15 when he moved from Notts County to Arsenal for a fee of £2m.
Wayne Rooney was 18 when he made his £20m move to Manchester United, who also paid £12m for an 18-year-old Cristiano Ronaldo.
Wenger is a huge admirer of the youngster and said he had tracked the player for more than two years.
He was delighted to have beaten off interest from Chelsea, Manchester United, Tottenham and Liverpool to his signing.
He said: "Theo Walcott is a very young player, but has already shown in his performances for Southampton that he is a talented player with huge potential.
"I also like the fact that he is a versatile player who is incredibly dedicated, shows great determination to succeed and is blessed with electric pace.
We have done everything to keep Theo at Southampton
Saints chairman Rupert Lowe
"We are delighted Theo has signed for the club and will make a fantastic addition to our squad.
"There is always a gamble when you take young players. But I was ready to take that gamble.
"We did things properly with Southampton, then the agents, the family, the player.
"He made his decision and we would have respected whatever he decided but we were keen to get him.
"He is comparable to Wayne (Rooney) at his age. You have to be cautious with comparisons.
"Rooney has made big steps forward and justified what people said about him and I feel Theo can do the same."
Southampton chairman Rupert Lowe told the club's website: "We are bitterly disappointed. The purpose of developing the best Academy in the country is not to sell scholars to larger clubs.
"We have done everything to keep Theo at Southampton. But his family and advisors are determined that he leave this club and go to Arsenal.
BBC Sport revealed on 13 January that Arsenal would be coming in with a big bid for Walcott, prompting Wenger to admit later that day that he was an admirer of the teenager's precocious talent.
He also let slip last weekend that a deal was almost done.
Referring to new arrivals Emmanuel Abedayor and Abo Diaby, he said Arsenal had signed "three players" but then changed that to "two-and-a-half".
Asked if he was referring to Walcott, the Arsenal manager said: "I would like to tell you."
Portsmouth boss Harry Redknapp, who gave Walcott his debut during his time at Southampton, believes Arsenal are getting a player of huge potential but warned against comparisons with Thierry Henry.
"The kid can run through puddles and not make a splash," he told the BBC. "He's lightning, he drifts over the ground and he's a fantastic talent. Wherever he goes I'm sure he's got a great future.
"Henry is as fantastic a footballer as you'll ever see and I hope this kid can go on and be a top player as well.
"But he is a million miles from Thierry Henry and if he ever gets anywhere near his standard he will be one of the best players we've ever seen in this country.
"He's got a long way to go to get there but, please God, he becomes a top player which I'm sure he can do."
Today, Arsenal have confirmed that they will be adding Theo Walcott to their side, after a 12.5 million pound deal was struck, on the premise that he pass rigorous medical examinations. Although the 16-year-old striker cannot sign until he turns 17 on 16 March, Wenger seems to see this only as a formality, as the player is set to be unveiled today at Highbury.

Although there are not many other details at this moment, it is thought that he will not be able to return to Southampton on loan, due to the hectic schedule that he will have at Arsenal. Whatever way the deal may turn out, it seems that Theo's family are with him whatever he does.