A much-changed Les Bleus side made a promising start, but 21-year-olds Forss and Valakari struck in the space of three first-half minutes to put Finland in command at the Stade de France on Thursday (AEDT).
France debutant Marcus Thuram struck the crossbar before Forss opened the scoring, but the world champion was toothless ahead of its Nations League encounters with Portugal and Sweden.
Finland, which also rung the changes, was solid after rocking Didier Deschamps' side with those two first-half goals from out of the blue and famously secured its first win over France, whose previous defeat came against Turkey in June 2019.
Thuram made a lively start on the left flank and almost opened the scoring when he rose high to head Lucas Digne's corner against the crossbar 15 minutes in, before volleying over unmarked from just eight yards out.
France was dominant but Finland took the lead against the run of play just before the half-hour mark, Forss driving into the penalty area to fire home with his left foot after Rasmus Karjalainen dispossessed Moussa Sissoko.
Things quickly went from bad to worse for Les Bleus, with Valakari cutting inside onto his left foot and picking his spot in the top-left corner with a stunning strike from around 20 yards out.
Finland was brimming with confidence and Steve Mandanda had to be alert to keep out Joni Kauko's effort with his feet early in the second half.
Antoine Griezmann, Anthony Martial and N'Golo Kante were introduced 12 minutes after the break, with Paul Pogba among the trio withdrawn, but France was running out of ideas.
Monaco full-back Ruben Aguilar made his debut off the bench and Griezmann firing wide in stoppage time when he ought to have scored summed up a frustrating night for France.