In this paper, we explore positivity bounds for the effective field theory~(EFT) of a single weakly coupled massive vector field. The presence of both mass and spin makes the crossing properties of the amplitudes vastly complicated -- we address this by parametrizing the amplitudes as products of a polarization matrix and a vector of appropriately chosen functions with simpler crossing properties. The resulting framework involves sum rules and null constraints that allows us to constrain any combination of low-energy observables, such as EFT amplitudes. By varying the value of the vector mass over the cutoff scale, some of our bounds asymptote to the bounds obtained in the context of photons and massless scalars. This work paves the way for future applications to e.g. non-abelian massive vectors, glueballs and theories with spin larger than one.
