We present high-precision spectroscopy of the ground-state hyperfine structure of HD^+ at 4~T. We determine the bound-electron g factor, g₄, ₁₎ₔ₍₃ = -2. 002\, 278\, 540\, 96 (40), to a relative uncertainty of 210^-10, the most precise determination of a bound-electron g factor of a molecular ion to date. The experimental value agrees with recently developed ab initio theory that now includes quantum-electrodynamical effects up to order α⁵ and has reduced the theoretical uncertainty by three orders of magnitude O. Kullie et al. , Phys. Rev. A 112 052813 (2025). In addition, we extract the scalar spin-spin interaction coefficients E₄~=~925\, 395. 758 (41) \, kHz (electron-proton) and E₅~=~142\, 287. 821 (22) \, kHz (electron-deuteron), which show a moderate tension with another state-of-the-art theoretical prediction M. Haidar et al. , Phys. Rev. A 106 042815 (2022).
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