The Sky Blues are believed to have netted close to $1.4million to sell the 20-year-old, who has been with the club since the age of 12.
Comfortable at centre back or as a defensive midfielder, Girdwood-Reich was one of Sydney's top performers in the A-League Men this season and his displays led to him earning caps for the Olyroos for the first time this year.
"I love this club, I started here as a 12-year-old so I'm sad and very emotional to be leaving but incredibly thankful for what Sydney FC has done for me over the past eight years," Girdwood-Reich said.
"I've always wanted to play in America and when this opportunity came up it really caught my eye.
"St Louis are a new club, they have a clear and exciting philosophy from which I can learn and I hope to show what I can do.
"To even have the possibility of playing against greats of the game such as Lionel Messi and Luis Suarez excites me."
Girdwood-Reich's sale follows several exits at Sydney FC as head coach Ufuk Talay looks to rebuild his squad.
Captain Luke Brattan is the highest-profile departure along with Slovakian winger Robert Mak.Â
"Jake has done extremely well and fully deserves this opportunity to progress his career," Talay said.
"He is proof that Australia's best young talent will get an opportunity at Sydney FC and potentially unlock a path to an overseas career."
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