A theory of nonlinear interactions of spin waves (SW) propagating in thin magnetic films is developed. We use the classical Hamiltonian formalism for SW in an unbounded ferromagnetic medium in combination with the theory of dipole‐exchange SW spectrum for magnetic films to derive a nonlinear Schro¨dinger equation for the SW envelope which gives a good qualitative description of the formation of the envelope soliton experimentally observed in yttrium‐iron garnet (YIG) films.