The 16-year-old from North Rhine-Westphalia was holidaying in the south Bavarian countryside in August when she found the treasure, which has a value of around €15,000, on the lake bed.
She then handed the find into the police, who then worked on identifying its owner.
According to German law the owner of the lost property has six months to come forward and claim it - otherwise the finder gets to keep it.
But no one came forward to claim the gold - meaning the teenager from North Rhine-Westphalia has just become a rather wealthy young woman, Focus reports.
It was clear from the beginning that an owner coming forward was unlikely.
Police noted back in August that the bar had been tampered with and that whoever had put it into the lake had scratched out the number by which it could normally be identified.
Rumours of Nazi treasure
Lake König. Photo: DPA
The fact someone tampered with the bar was not the only curious thing about the find.
Lake König happens to lie a mere three kilometres from Hitler’s summer residence the Berghof, where he spent much of the Second World War.
Rumours have swirled for decades about a stash of gold hidden by the Nazis at the bottom of a Bavarian lake, and Lake König has always been regarded as a likely candidate, Die Welt reports.
But local police have made clear the gold bar does not come from the Nazi period.
“There is no hidden treasure at the bottom of the lake,” a spokesperson told Die Welt in August.
Focus also reports that in the meantime police divers have searched the lake bottom for more valuables, without success.
“If our specialists didn’t find anything there, no one else will find anything either,” a spokesperson said.
A spokesperson for Berchtesgaden police also told The Local that the gold bar was produced after 1945.
But who the gold original belonged to remains a mystery. Although police specialist were able to identify the serial number, they couldn’t trace it back to an owner.