Using p p collision data corresponding to an integrated luminosity of 5.4 fb − 1 collected with the LHCb detector at a center-of-mass energy of 13 TeV, the B 0 → D − D + K + π − decay is studied. A new excited D + s meson is observed decaying into the D + K + π − final state with large statistical significance. The pole mass and width, and the spin parity of the new state are measured with an amplitude analysis to be m R = 2591 ± 6 ± 7 MeV , Γ R = 89 ± 16 ± 12 MeV , and J P = 0 − , where the first uncertainty is statistical and the second systematic. Fit fractions for all components in the amplitude analysis are also reported. The new resonance, denoted as D s 0 ( 2590 ) + , is a strong candidate to be the D s ( 2 1 S 0 ) + state, the radial excitation of the pseudoscalar ground-state D + s meson.