It's a first senior individual medal at a major championships for Kosonen, who finished fourth or fifth at the previous four major outdoor events.
At the world championships in Tokyo last year, Tervo qualified fifth only to fail to register a throw in the final, and that may well have been playing on her mind as she began this final with a no throw. Kosonen, fourth in Tokyo, brushed the netting with her first attempt of 69.76m.
That put her in fourth ahead of her second throw - a much cleaner effort that sailed past her opening mark and dropping at 74.97m to take her into first place. Kosonen was still in gold medal position as she entered the circle for her third throw, which went further still - 76.28m from the 23-year-old.
But for Tervo, things barely improved - she got a score on the board with her second throw, 67.74m, but another failure with her third effort meant she was in 11th at the halfway stage, not making the cut for the second half of the final with her medal chances in tatters.
As the competition reached its climax, Kosonen was unable to replicate her third-round throw with her next two attempts, but her lead barely came under threat, and after reigning European champion Sara Fantini of Italy was unable to improve on 74.35m with her final attempt, the gold belonged to the Finn. Bronze went to Beatrice Nedberge Llano of Norway (72.88m).
It's Finland's first European championship gold since 2022, when Topi Raitanen won the men's 3000m steeplechase and Wilma Murto took the women's pole vault crown.
110m hurdlers bow out in semi-finals
Elmo Lakka was out in lane nine for the second semi-final in the 110m hurdles, and he got out of the blocks well, only to hit the first barrier and lose plenty of his early momentum. In the end, he clocked a time of 13.89, slightly slower than his first round time, and sixth place was not enough to reach the final.
Lane nine was also where Santeri Kuusiniemi lined up for the final semi-final, and again it brought the Finns no luck as a slower reaction to the gun followed by a clip of the second hurdle contributed to a 13.96 run, also down on his heat time, and a seventh-placed finish.
No finalists in discus or 400m hurdles
In the morning session, a best throw of 51.96m was only good enough for 13th in group B of the women's discus qualification for Helena Leveelahti, putting her well outside the required distance to reach the final.
And one of the last out on the track in the morning was Antti Sainio, running in semi-finals of the 400m hurdles. After a season's best 49.70 in the heats, Sainio began strongly on one of the inside lanes, but ran out of steam in the final 100m, finishing last in 50.64.