Let (M,π) be a Poisson
manifold. A Poisson submanifold P ⊂ M gives rise to a Lie algebroid AP→ P.
Formal deformations of π around P are controlled by certain cohomology groups
associated to AP. Assuming that these groups vanish, we prove that π is
formally rigid around P; that is, any other Poisson structure on M, with
the same first-order jet along P, is formally Poisson diffeomorphic to π.
When P is a symplectic leaf, we find a list of criteria that are sufficient for
these cohomological obstructions to vanish. In particular, we obtain a formal
version of the normal form theorem for Poisson manifolds around symplectic
leaves.