The 31-year-old Everton striker launched his professional career with the Saints, making his senior debut at the age of 16 under then manager Harry Redknapp.
However, he has found himself surplus to requirements at Goodison Park under Carlo Ancelotti and jumped at the chance to return.
He said: “I had a few opportunities from other clubs, but as soon as Southampton came in for me, it’s part of me and has made me into the player I am. It’s in my heart.
“I will probably cry, I bet, when I first play, and it means so much. All the people that made it happen, I can’t thank them enough. Honestly, I’m so pleased.
“I was seriously lost for words when Southampton came on the table, and for me it was such an easy decision.
“To get the chance to work with the manager as well and these young players that are coming through, I feel like I’ve made the right choice.”
Walcott emerged from the ranks at Southampton to become the club’s youngest ever senior player, making his first start at Leeds in October 2005.
Just months later, he was heading for Arsenal in a £5million January move and the England international spent 12 years in North London before the Toffees invested £20million in his services in January 2018.
Ancelotti’s summer spending spree saw him slip down the pecking order and prompted his temporary exit, but the man himself is confident he still has much to offer.
Walcott told Sky Sports News: “I want to work hard and try to get Southampton to go to that next level. I feel like as well with my old head, I suppose now, I can help the young players come through as well.
“I’m here to win, to fight, so I still feel like I’ve got a lot in me.”
Saints boss Ralph Hasenhuttl is confident Walcott will prove an ideal mentor for the club’s emerging professionals.
He said: “There is a lot of history between this club and Theo, so I know how special this will be for him and for our supporters.
“But what is most important to me is that we have signed a player who can help our squad and give us another strong option in our attack.
“Theo is someone who has all the qualities to fit well into our team and our style of play, and I know from speaking to him that he understands what we want and that he is excited to be a part of what we are doing here.
“His experience and influence will also be important for our young players. The idea is not to block their development, but for this to make it faster. From this professional, they can learn and grow as players for the future of our club.”
