We develop a theory of non-linear cosmological perturbations at superhorizon scales for a scalar field with a Lagrangian of the form P(X,φ), where X = −^μφ_μφ and φ is the scalar field. We employ the ADM formalism and the spatial gradient expansion approach to obtain general solutions valid up to the second order in the gradient expansion. This formulation can be applied to, for example, DBI inflation models to investigate superhorizon evolution of non-Gaussianities. With slight modification, we also obtain general solutions valid up to the same order for a perfect fluid with a general equation of state P = P(ρ).