Justyna Święty-Ersetic competed in the
4 × 400 metres relay at the
2012 London and
2016 Rio Summer Olympics. She won the bronze medal at the
2017 World Championships in the event. At the
2018 European Championships in Berlin, she won an individual gold and later also anchored the Polish squad to the win despite the fact that both finals were separated by less than two hours. At the
2020 Tokyo Olympics she represented Poland in the
4 × 400 m mixed relay, winning gold alongside
Karol Zalewski,
Natalia Kaczmarek and
Kajetan Duszyński. A few days later, Święty-Ersetic anchored the
women's 4 × 400 m relay (Kaczmarek,
Iga Baumgart-Witan and
Małgorzata Hołub-Kowalik), sealing a silver for the group. In March 2022, Święty-Ersetic won for the second time
World Athletics Indoor Tour in her specialist distance after her 2020 victory, breaking the meeting record in
Madrid. She also competed for
Poland at the 2024 Summer Olympics. Her personal bests in the 400 metres are 50.41 seconds outdoors (
Berlin 2018), and 51.04 seconds indoors (
Toruń 2022). She was part of teams which set national records for the women's and mixed relay (both
Tokyo 2021), and women's indoor relay (
Birmingham 2018). ==Personal bests==