The Sinai billiard map T on the two-torus, i.e., the periodic Lorentz gas, is a discontinuous map. Assuming finite horizon, we propose a definition h * for the topological entropy of T . We prove that h * is not smaller than the value given by the variational principle, and that it is compatible with the definitions of Bowen using spanning or separating sets. If h * ≥ log 2 (our actual condition is weaker), then we get more: First, using a transfer operator acting on a space of anisotropic distributions, we construct an invariant probability measure µ * of maximal entropy for T (i.e., hµ * (T ) = h * ), we show that µ * has full support and is Bernoulli, and we prove that µ * is different from the smooth invariant measure except if all non grazing periodic orbits have multiplier equal to h * . Second, h * is compatible with the Bowen-Pesin-Pitskel topological entropy of the restriction of T to a non-compact domain of continuity. Last, applying results of Lima and Matheus, the map T has at least Ce pnh * periodic points of period pn for all n and some p ≥ 1.