The main result of this paper is the following: any 'weighted' Riemannian manifold (M, g, µ) -i.e. endowed with a generic non-negative Radon measure µ -is 'infinitesimally Hilbertian', which means that its associated Sobolev space W 1,2 (M, g, µ) is a Hilbert space.We actually prove a stronger result: the abstract tangent module (à la Gigli) associated to any weighted reversible Finsler manifold (M, F, µ) can be isometrically embedded into the space of all measurable sections of the tangent bundle of M that are 2-integrable with respect to µ.