Finding intermediate-mass black holes (IMBHs) and measuring their masses and spins are key to understanding massive black hole formation. White dwarf (WD)-IMBH binaries provide a unique probe because they emit both electromagnetic radiation and gravitational waves (GWs), thereby conveying richer information. However, such multimessenger sources often enter the regime of strong gravity, where existing models often fail to capture their relativistic dynamics. Here, we develop a fully relativistic model for the tidal response of a WD close to an IMBH, and use it to study the secular orbital evolution as well as the GW signal. We find that for IMBHs more massive than ${10}^5{M}_{\odot }$, tidal interaction becomes relativistic and sensitive to IMBH spin. The interaction generally dissipates binary orbital energy and angular momentum, but due to relativistic frame rotation, which reduces phase coherence across pericenter passages, the orbit-averaged tidal dissipation rate can be suppressed by up to $\sim 50\%$ relative to Newtonian predictions. Including tidal dissipation leads to more rapid damping of the orbital eccentricity, to the extent that the pericenter distance may even increase over time, potentially explaining quasiperiodic eruptions and secular orbital period growth. Such tidal effect accumulates into measurable phase and amplitude deviations in the GW signal. For typical space-based observations, the GW waveform mismatch can reach $\mathcal{O}(0.1)$ within six months. Our results indicate that relativistic tidal dissipation is both dynamically important and observationally essential for reliably predicting the multimessenger signals of WD-IMBH systems.

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• General relativity
• Gravitational waves
• Massive compact halo objects
• Astronomical black holes