Topological defects can act as local impurities that seed cosmological phase transitions. We will focus on domain walls and discuss their impact on the electroweak phase transition in the minimal extension of the SM including a scalar singlet (xSM), odd under a Z_2 symmetry. When the transition is two-step, the early breaking of the Z_2 implies the formation of domain walls, which can then act as seeds for the second and final step. The rate of the seeded phase transition can be evaluated within a 3d theory on the domain wall surface, and it is generically faster than the standard homogeneous tunneling. We will comment on the phenomenological implications of this type of transition.