The former England winger first signed for Saints at the age of And is back on loan from Everton, 14 years after leaving the club.
On Monday night, his return was shown as part of a one-season loan agreement.
Carlo Ancelotti had presented two outings to the 31-year-old this season and the early start of the Toffees season warned that additional opportunities would be limited.
Southampton took the opportunity to raise the joy of the climb to the front line of Ralph Hasenhuttl.
Speaking after Southampton showed the deal, Walcott admitted that he felt the excitement of the moment.
He told the club’s website: “I’ve had a few opportunities at other clubs, but as soon as Southampton arrived for me, it’s a component of me and it’s made me the player I am. It’s in my heart. “
“I’ll probably cry, handsome, when I first play, and that means a lot. All the other people who made this happen, I can’t thank them enough. Honestly, I’m very happy.
“I was speechless when Southampton came to the table, and for me it was such a simple decision.
“Having the opportunity to work with the coach and the young players who are coming, I feel like I made the right decision. “
At 16 and 143 days old, Walcott was the club’s youngest player when he debuted in August 2005.
The Berkshire-trained winger made 22 appearances for the Saints before Arsenal picked him up for a down payment of five million pounds, six months after his debut.
This encouraged England national teamer Sven Goran Eriksson to make the bold, albeit ultimately failed, decision to take Walcott to the 2006 World Cup.
After more than a decade and two FA Cup victories at Arsenal, the winger went to Everton and scored 11 goals in matches at Goodison Park.
And he hopes to make his debut for the time being against Chelsea on 17 October, with a return of St Mary’s against Everton a week later.
Southampton technician Hasenhuttl said: “There’s a lot of history between this and Theo, so I know how special it will be for him and our followers.
“But what’s most vital to me is that we’ve hired a player who can our team and gives us some other strong choice in our attack.
“Theo is the one who has all the qualities to have good compatibility with our team and our taste for the game, and I know by talking to him that he understands what we need and is extremely happy to be a part of what we do here.
“Your delight and influence will also be for our young players. The concept is not to block its development, but to do it faster. From this professional, they can be informed and grow as players for the long term of our club. “
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