Within the standard propagation scenario, the flavor ratios of high-energy cosmic neutrinos at neutrino telescopes are expected to be around the democratic benchmark resulting from hadronic sources, (1 : 1 : 1) ⊕ . We show how the coupling of neutrinos to an ultralight dark matter complex scalar field would induce an effective neutrino mass that could lead to adiabatic neutrino propagation. This would result in the preservation at the detector of the production flavor composition of neutrinos at sources. This effect could lead to flavor ratios at detectors well outside the range predicted by the standard scenario of averaged oscillations. We also present an electroweak-invariant model that would lead to the required effective interaction between neutrinos and dark matter.Introduction.-Although cosmological observations have determined the contribution of dark matter (DM) to the energy budget of the Universe with an outstanding precision, the nature of the particles making up this component of the Universe is still unknown. In particular, the mass, spin and couplings of DM particles have not been determined yet. A lower bound on the mass (m DM ) comes from the de Broglie wavelength of the DM particle, λ dB = 2π/(m DM v), which is required to be smaller than the size of dwarf galaxies. Ultralight bosonic DM with a mass close to this bound, ∼ 10 −22 −10 −21 eV, has gained popularity (see, e.g., Refs. [1-6] for reviews), as it can address the small structure problems that the canonical cold DM scenario suffers from [7][8][9]. Recent studies of rotation curves of nearby galaxies [10], of dwarf galaxies [11][12][13], the comparison between the predictions of hydrodynamical simulations and Lyman-α observations [14][15][16][17], and analyses of cosmological data [18][19][20] have set lower bounds of ∼ 10 −21 eV on m DM .As long as the de Broglie wavelength, λ dB , is much larger than the average distance between DM particles (∼ n