  The strange quark contribution to proton's spin ($\Delta s$) is a fundamental
quantity that is poorly determined from current experiments. Neutrino-proton
elastic scattering (pES) is a promising channel to measure this quantity, and
requires an intense source of low-energy neutrinos and a low-threshold detector
with excellent resolution. In this paper, we propose that neutrinos from a
galactic supernova and their interactions with protons in large-volume
scintillation detectors can be utilized to determine $\Delta s$. The spectra of
all flavors of supernova neutrinos can be independently determined using a
combination of DUNE and Super-(Hyper-)Kamiokande. This allows us to predict pES
event rates in JUNO and THEIA, and estimate $\Delta s$ by comparing with
detected events. We find that the projected sensitivity for a supernova at 1
kpc (10 kpc), is approximately $\pm 0.01$ ($\pm 0.15$). Interestingly, the
limits from a nearby supernova would be comparable to the results from lattice
QCD, and better than polarized deep-inelastic scattering experiments. Using
supernova neutrinos provides a true $Q^2\rightarrow 0$ measurement, and thus an
axial-mass independent determination of $\Delta s$.
