A novel unknown input observer (UIO) using sliding modes is proposed for a general class of uncertain, nonlinear systems. The system generalization over-comes requirement of observability matching condition for the uncertainties and also does not rely on any transformation techniques, typically used for nonlinear systems. The UIO philosophy of disturbance decoupling is used for compensating the matched disturbances and overall robustness is engineered using sliding modes. Fault detection is proposed using the estimations. The efficacy of the method is illustrated through simulations on a quadruple tank process (QTP) plant. © 2021 IEEE.