Interference observed in a double-slit experiment most conclusively demonstrates the wave properties of particles. We construct a quantum mechanical double-slit interferometer by rovibrationally exciting D2 (v=2, j=2) molecules in a biaxial state using Stark-induced adiabatic Raman passage. In D2(v=2, j=2)→D2(v=2, j'=0) rotational relaxation via a cold collision with ground state He, the entangled bond axis orientations in the biaxial state act as two slits generating two indistinguishable quantum mechanical pathways connecting initial and final states of the colliding system. The interference disappears when we decouple the two orientations of the bond axis by separately constructing the uniaxial states of D2, unequivocally establishing the double-slit action of the biaxial state. This double slit opens new possibilities in the coherent control of molecular collisions.