QPR defender Steven Caulker is to undergo a medical at Southampton on Wednesday ahead of a season-long loan move.
Brentford midfielder Jonathan Douglas has been told his time at the club is over by new boss Marinus Dijkhuizen.
Saints revealed the Perth-born former Scotland Under-21 international had signed a two-year contract following the expiry of his Easter Road deal.
The 23-year-old, who can also play right-back, made more than 150 appearances for Hibs, including 38 last season, but missed out on a place in the William Hill Scottish Cup final squad for the second year running.
Wotherspoon is manager Tommy Wright's second signing after he renewed 20-year-old Gwion Edwards' loan deal from Swansea until the end of the year.
The midfield pair's arrival comes days after St Johnstone agreed a deal with Ross County to allow Richie Brittain to renege on the pre-contract he signed with the Europa League qualifiers in January.