The 31-year-old had taken over at Goodison Park after the summer arrival of James Rodriguez, who played 12 minutes in his first four games.
The winger now left existing Premier League leaders to sign for Southampton, where he rose through the ranks before moving to Arsenal at the age of 16 in 2006.
“I had some opportunities at other clubs, but as soon as Southampton came here for me, it’s a component of me and has made me the player I am. It’s in my heart,” Walcott told the club’s official website.
“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 ability to paint with the coach and the young players coming in, I feel like I made the right decision. “”
Coach Ralph Hasenhuttl added: “There’s a lot of history between this club and Theo, so I know how special it will be for him and our fans. But the most important thing for me is that we’ve hired a player who can help. our team and give us a 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 vital for our young players. The concept is not to block its development, but to do it faster. From this professional, they will be informed and grow as players in the long run of our club. “
Walcott scored goals in 23 appearances in his first season with Southampton.