The 29-year-old resident of Poltava defeated Nurbek Oralbay of Kazakhstan in the final, Ukrinform reports.
Khyzhniak took silver at the Olympic Games in Tokyo in 2020.
President Volodymyr Zelensky congratulated the Ukrainian boxer on his victory.
"Another gold medal for Ukraine at the Olympics in Paris! Boxer Oleksandr Khyzhniak has taken the first place. Thank you for your strength, your confidence, and for this important victory! We continue to support all our athletes. We cheer for Ukraine, we cheer for Ukrainians!" Zelensky wrote on X.
In official medal standings, Ukraine is in the top 20 with three gold, two silver and three bronze medals.
Read also: Ukraine’s Beleniuk, Nasibov push through to semi-finals in Greco-Roman wrestling at Paris GamesThe Olympic Games in the French capital will end on August 11.
Photo: Ukraine's National Olympic Committee