The only anticipated resonant contributions to B + → D + D − K + decays are charmonium states in the D + D − channel. A model-independent analysis, using LHCb proton-proton collision data taken at center-of-mass energies of √ s = 7 , 8, and 13 TeV, corresponding to a total integrated luminosity of 9 fb − 1 , is carried out to test this hypothesis. The description of the data assuming that resonances only manifest in decays to the D + D − pair is shown to be incomplete. This constitutes evidence for a new contribution to the decay, potentially one or more new charm-strange resonances in the D − K + channel with masses around 2.9 GeV / c 2 .