The 23-year-old, who started his career with Celtic, has agreed a two-year contract at McDiarmid Park.
Wotherspoon, who supported Saints as a youngster, made over 140 appearances for Hibs during a four-year stint, scoring the winner in his final game against Dundee in May.
Meanwhile, St Johnstone have also renewed their loan deal with Swansea for Gwion Edwards.
The 20-year-old midfielder has joined for the first half of the season with a view to extending the deal.
Edwards joined the McDiarmid Park side on loan in January and new manager Tommy Wright was sufficiently impressed to offer him another chance to become a first-team regular.