A probabilistic Boolean network (PBN) is a collection of Boolean networks endowed with a probability structure describing the likelihood with which a constituent network is active at each time step. This paper proposes a definition of bisimulation for PBNs. The notion is inspired by the analogous notions for probabilistic chains and for stochastic linear systems. Necessary and sufficient conditions to check the proposed notion are derived, model reduction of PBNs via bisimulation is addressed, and a discussion on the use of probabilistic bisimulation for optimal control design is given. The present results extend the theory of bisimulation known for deterministic Boolean networks to a stochastic setting.