Walcott became the youngest player to feature for Southampton when he made his first-team debut in August 2005, at only 16 years and 143 days, and his instant impact at St Mary’s helped him secure a move to Arsenal five months later.
British media reports said several Premier League clubs were interested in signing Walcott, who has struggled for first-team opportunities under Everton manager Carlo Ancelotti.
But the 31-year-old said it was an easy decision to make once Southampton came calling.
“I will probably cry, I bet, when I first play, and it means so much,” Walcott said in a statement.
“Southampton came in for me, it’s part of me and has made me into the player I am. It’s in my heart. 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.”