We define Lie and Courant algebroids on Fréchet manifolds. Moreover, we construct a Dirac structure on the generalized tangent bundle of a Fréchet manifold and show that it inherits a Fréchet Lie algebroid structure. We show that the Lie algebroid cohomology of the B-cotangent bundle Lie algebroid of a weakly symplectic Fréchet manifold M is the Lichnerowicz-Poisson cohomology of M .