A measurement of the photon polarization in radiative penguin B decays provides a test of the Standard Model and a probe for New Physics, that can lead to a deviation from the Standard Model prediction of left-handed photons in b → s γ . We propose a new method to measure the photon polarization using the baryonic decay B - → Λ p ¯ γ . The P -violating Λ -hyperon decay allows a measurement of the Λ helicity to be performed, which can be uniquely related to the photon polarization in a model-independent way. The B - → Λ p ¯ γ decay was recently measured to have a large branching fraction providing a possibility to get meaningful results with the data already available at LHC and B-factory experiments. An increase of the B -meson sample at high luminosity LHC experiments and Belle II should provide a really stringent test by using this method already in the near future.