After Schallenberg, 26, made a birdie putt on the par-5 36th hole, the pressure was on Kim to birdie for the victory at Pumpkin Ridge Golf Club.
She sunk it from 5 feet out.
'Wow!' she said after high-fiving her caddie and finding her father in the gallery for a hug.
The championship match, on the 6,380-yard, par-71 Witch Hollow Course, featured very different players: Kim is a carefree teenager who didn't know going in that the Amateur was a 'big' tournament, while Schallenberg is consistent and steady.
Schallenberg got off to a fast start, going as much as 5-up on the opening 18 holes. But she lost steam and lost the last three holes of the morning and was 2-up at the break.
Kim was loose, with words of inspiration -- including 'Trust' -- written in magic marker on her wrist.
She and her caddie would often sit on the fairway while waiting for her shot, taking a break from temperatures in the low 90s.
In the afternoon, Schallenberg struggled with three straight shots into bunkers on the par-5 No. 25 hole, Kim took over with a birdie putt on the par-4 No. 26, pulling the match to all square.
Kim went 2-up on the 31st hole with a par putt, and held on for the lead.
Schallenberg briefly played at the University of Oregon in 2000 but quit because she had a fear of flying. She returned to Germany, where she apprenticed to be a bank clerk but found it 'too boring,' and is now studying international business.
She won the 2005 and 2006 International German Amateur Championships, but has played infrequently in the United States.
Kim, who has a mouthful of braces and says her hobby is sleeping, has been living recently in Arizona to be able to participate in more tournaments.
Kim defeated 15-year-old Lindy Duncan of Plantation, Fla., 1-up in Saturday's semifinals to make it to the finals, while Schallenberg downed 21-year old Texan Stacy Lewis on the 19th hole.
Earlier in the week, Paige Mackenzie was the medalist in stroke-play qualifying.
Last year's Amateur was won by 17-year-old Morgan Pressel at the Ansley Golf Club's Settindown Creek course in Roswell, Ga.
Pumpkin Ridge Golf Club was the site of the U.S. Women's Open in 1997 and 2003. Tiger Woods won the 1996 U.S. Amateur at the club.