The problem of diffraction of a plane wave incident on a thin semi-infinite chiral slab is solved in an explicit form and the effect of chirality on diffraction phenomenon is investigated for the first time. The thin layer is simulated by appropriate coupled transition boundary conditions of the first order with respect to the thickness of the slab. Application of the law of conservation of energy enables us to prove the uniqueness of the solution and to give some sufficient conditions for the parameters of the problem securing the unique solvability. The problem in question is solved by means of the Sommerfeld integrals and by some minor modification of the Maliuzhinetz technique. The asymptotics of the far field is studied and some numerical results for the diffraction coefficients axe discussed.