Rainbow structures in the scattering angular distribution play an important role in deepening our understanding about the elastic and rotationally inelastic collisions of atoms/molecules. Reported here is the discovery of a rainbow in a chemical reaction. At E-c= 4.3 kcal.mol(-1) one of the correlated product pairs in the F + CH3D reaction, (v(HF), v(CH2D)) = (3, 0(0)), displays a distinct bulge in angular distribution. We showed that the bulge originates predominantly from the low-j states of the HF(v = 3) products. Heuristic considerations led us to propose that such a bulge could be regarded as a signature for rainbow scattering. The underlying mechanism for its occurrence in this nearly thermoneutral product pair is ascribed to a delicate interplay of the attractive and repulsive parts of interactions in the vicinity of the transition state. In a sense, the situation bears striking, similarity to the more familiar elastic rainbow, thus coined "reactive rainbow".