This paper presents a probabilistic model validation methodology for nonlinear systems in time-domain. The proposed formulation is simple, intuitive, and accounts both deterministic and stochastic nonlinear systems with parametric and nonparametric uncertainties. Instead of hard invalidation methods available in the literature, a relaxed notion of validation in probability is introduced. To guarantee provably correct inference, algorithm for constructing probabilistically robust validation certificate is given along with computational complexities. Several examples are worked out to illustrate its use. , . . . , b s ; x) stands for generalized hypergeometric function. The symbols N (., .), U (.), and A (.) are used for normal, uniform and arcsine distributions, respectively. We use the notation ξ 0 (.) to denote the joint PDF over initial states and parameters. ξ (., t) and ξ (., t) denote joint PDFs over instantaneous states and parameters, for the true and model dynamics, respectively. Similarly, η (., t) and η (., t), respectively denote joint PDFs over output spaces y and y at time t, for the true and model dynamics. The symbol x is used to denote the extended state vector obtained by augmenting the state (x) and parameter (p) vectors. We use χ to denote indicator function and # to denote cardinality. Unless stated otherwise, δ (.) stands for Dirac delta. The symbol I denotes the -by-identity