In this paper we consider the fully nonlinear parabolic free boundary problemwhere K > 0 is a positive constant, and Ω is an (unknown) open set.Our main result is the optimal regularity for solutions to this problem: namely, we prove that W 2,n x ∩ W 1,n t solutions are locally C 1,1x ∩ C 0,1 t inside Q1. A key starting point for this result is a new BMO-type estimate which extends to the parabolic setting the main result in [4].Once optimal regularity for u is obtained, we also show regularity for the free boundary ∂Ω ∩ Q1 under the extra condition that Ω ⊃ {u = 0}, and a uniform thickness assumption on the coincidence set {u = 0}, A.