Using pp 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 B0 → D−D+K+π− decay is studied. A new excited Ds+ 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 mR = 2591 ± 6 ± 7  MeV, ΓR = 89 ± 16 ± 12  MeV, and JP = 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 Ds0(2590)+, is a strong candidate to be the Ds(21S0)+ state, the radial excitation of the pseudoscalar ground-state Ds+ meson.