A Feshbach resonance occurs when the energy of two interacting free particles comes to resonance with a molecular bound state. When approaching this resonance, dramatic changes in the interaction strength between the particles occur. Feshbach resonances have been an essential tool to control the atom interactions, which can even be switched from repulsive to attractive [1-4]. Thanks to Feshbach resonances many effects in ultracold atomic gases could be explored [5, 6]. Here we demonstrate a Feshbach resonance based on polariton spinor interactions that characterize the fundamental interaction process in polariton quantum systems. We show the clear enhancement of attractive interactions and the prompt change to repulsive interaction by tuning the energy of two polaritons with anti-parallel spins across the biexciton bound state energy. A mean field two-channel model quantitatively reproduces the experimental results. This observation paves the way for a new tool to tune the polariton interactions and to move forward into quantum correlated polariton physics.

supplementary material included. available at http://www.nature.com/nphys/journal/v10/n7/full/nphys2999.html