The 23-year-old completed his move from Hibernian yesterday after making more than 150 appearances for the Edinburgh club, including 38 last season.
The Perth-born midfielder supported St Johnstone as a boy and was once on their books before continuing his football education at Celtic and Hibs.
Wotherspoon, who has signed a two-year deal, told the club's official website: "I grew up supporting St Johnstone and started off here playing for the under-12s so it's great to be back."
On the reaction of his family and friends, Wotherspoon said: "I've always wanted to sign for Saints so they are delighted. I'll have plenty of family support at games."
Manager Tommy Wright added: "We're delighted to get David signed up. It has happened quickly because we didn't think we'd be able to get him, but I spoke to him and it was obvious how much he wanted to sign for the club."
Wotherspoon is set to make his debut in the Europa League on July 18 as Saints await the winner of the clash between Crusaders and Rosenborg.
The former Scotland Under-21 international said: "I was at the game against Monaco in 1999 with my dad and my brother and that game is one of my best football memories.
"It really was fantastic to see St Johnstone play against players like Fabien Barthez, David Trezeguet and all the other stars they had in that team.
"Saints got a draw and watching games like that are what makes you want to play football.
"It will be a real honour to play in a Europa League for the club, but first I have to get settled in and work hard to show the manager I deserve to be in the team."