Recent studies have shown that rotating black holes can undergo spontaneous scalarization, leading to deviations from general relativity (GR) in the strong-field regime. We present the first nonperturbative calculation of the quasinormal modes of scalarized Kerr black holes in Einstein-scalar-Gauss-Bonnet gravity, without assuming small spin or weak coupling. Our results reveal a universal splitting of the fundamental $l=m=2$ mode into axial-led, polar-led, and scalar-led branches, breaking the isospectrality characteristic of Kerr black holes. This splitting offers distinct signatures in the ringdown phase of gravitational wave signals and provides a new avenue to test gravity in the strong-field regime. Our findings open the possibility of probing beyond-GR physics using precision measurements of black hole ringdowns in upcoming gravitational wave observations.

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• Alternative gravity theories
• Classical black holes
• Gravitational wave sources