Kuś clocked 51.06 seconds in the final in the early hours of Saturday morning Warsaw time, finishing ahead of American Ataja Stephanie-Vazquez in 51.36 and Guyana's Tianna Springer in 51.76, Polish state news agency PAP reported.
"I thought winning a medal would be the safe option, while I didn’t expect gold at all," Kuś said in a statement released by the Polish Athletics Association (PZLA), the national governing body for the sport.
"During the race, though, I felt I was in a medal position. I knew I could fight for it and kept fighting until the very end."
Kuś said she thought she would finish second as she entered the final straight but managed to overtake Stephanie-Vazquez in the closing metres.
"I really wanted to win that gold medal and, with the last of my strength, I managed to snatch it in the final metres," said Kuś, who also set a Polish U20 record in the women's 400m.
Kuś, who hails from Olsztyn, northern Poland, is the daughter of former Poland international soccer player Marcin Kuś.
Marcin Kuś, pictured in 2011. Photo: AntalyasporFC, CC BY-SA 3.0 <https://creativecommons.org/licenses/by-sa/3.0>, via Wikimedia Commons
She will receive her medal on Saturday evening local time before travelling directly to Birmingham, England, where the European senior championships begin on Monday.
"I’ve already achieved my goal for this season by winning gold at the U20 worlds, so now I can approach my next competition with a calm mind," Kuś said.
The U20 world championships in Eugene run until Sunday.
The European senior championships in Birmingham will feature more than 1,600 athletes from 49 countries.
In 2024, Kuś won gold in the 400m at the European Athletics U18 Championships in Banská Bystrica, Slovakia. She clocked 51.89 seconds, breaking both the Polish U18 record and the competition record, which had stood since 2016.
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Source: TVP Info, PAP