Great Britain’s Georgia Hunter Bell finished second in 3:57.72. The United States’ Nikki Hiltz took bronze in 3:58.16.
Kazimierska, racing in front of a strong Polish presence at the Gyula Zsivótzky National Athletics Centre, absorbed pressure down the home straight and refused to yield as Hunter Bell closed to within 0.17 seconds.
The result adds another major medal to a meeting that has already seen Mondo Duplantis win the men’s pole vault with 6.20m and Jakob Ingebrigtsen take the men’s 5,000m in 12:59.24.
Hunter Bell, a leading figure on the global 1,500m circuit, produced a fierce finish but could not overhaul the Pole. Hiltz completed an all-international podium in 3:58.16.
The three-day World Athletics Ultimate Championship, which offers a $10 million prize pool across 28 events, continues in Budapest through Sunday.