Valakari started his coaching career in his homeland and guided SJK to top-flight promotion before winning their first Veikkausliiga title in 2015.
The former Derby County midfielder also claimed both domestic cups during his time at SJK before a three-year stint at Tromso in Norway.
Valakari, who played more than 100 games for Motherwell and won over 30 Finland caps, would return home and claim consecutive Finnish Cups with KuPS.
Then came a move to Latvia, where a short stint at FK Auda was followed by a switch last year to Riga, where he won the Latvian Supercup.
St Johnstone owner Adam Webb said: “Simo has a remarkable vision for the future. His experience, enthusiasm and creativity will serve us well in the years ahead.”
As part of the appointment, Gus MacPherson will now become head of football operations, scouting and recruitment.
And St Johnstone also confirmed the interim management team of Andy Kirk and Alex Cleland will remain in charge for this weekend’s game against Rangers at Ibrox.