  If ultra-light dark matter (ULDM) exists and couples to neutrinos, the
neutrino oscillation probability might be significantly altered by a parametric
resonance. This resonance can occur if the typical frequency of neutrino
flavor-oscillations $\Delta m^2/(2E)$, where $\Delta m^2$ is the mass-squared
difference of the neutrinos and $E$ is the neutrino energy, matches the
oscillation frequency of the ULDM field, determined by its mass, $m_\phi$. The
resonance could lead to observable effects even if the ULDM coupling is very
small, and even if its typical oscillation period, given by
$\tau_\phi=2\pi/m_\phi$, is much shorter than the experimental temporal
resolution. Defining a small parameter $\epsilon_\phi$ to be the ratio between
the contribution of the ULDM field to the neutrino mass and the vacuum value of
the neutrino mass, the impact of the resonance is particularly significant if
$\epsilon_\phi m_\phi L\gtrsim 4$, where $L$ is the distance between the
neutrino source and the detector. Such parametric resonance can improve the fit
to the KamLAND experiment measurements by about $3.5\,\sigma$ compared to
standard oscillations. This scenario will be tested by the JUNO experiment.
