Tears flowed as ninth seed Korneeva prevailed 6-7 (2-7) 6-4 7-5 at Rod Laver Arena on Saturday.
The exhausted players, both just 15 years old, embraced at the net after a battle that lasted three hours, 17 minutes.
Korneeva wasted two championship points in the 10th game of the third set - the second of those with a double-fault - as Andreeva broke to stay in the match.
But Korneeva broke straight back and won her final service game to love, clinching the championship in her first attempt at a junior grand slam.
"It's not our last battle," Korneeva told her emotional opponent post-match.
"We will have a lot of good matches when you will win, when I will win ... it was a hard battle."
Koreenva, who idolises tennis legends Serena Williams and Rafael Nadal, is the first Australian Open girls' champion from Russia since Elizaveta Kulichkova in 2014.
She also follows in the footsteps of fellow Russians Ksenia Pervak (2009) and two-times winner Anastasia Pavlyuchenkova (2006-07).
Korneeva also joins a girls' title honour roll that includes Evonne Goolagong Cawley (1970) and fellow former world No.1 Victoria Azarenka (2005), who both went on to claim the women's crown on multiple occasions.
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