A new technique is presented in this paper for the optimal design of digital PID controllers with closed-loop and intersample behaviors taken into account. This method is based on minimizing the integral of the squared error (ISE) of the digital controlled system subject to a unitstep disturbance input. To ensure global optimality of the designed digital controller, a differential evolution algorithm is applied along with an analytic approach to evaluating the ISE cost functional in order to search for the controller parameters. Also, a formula is derived for obtaining the optimal set-point input weighting factors in order to achieve better reference tracking performance. To illustrate the proposed digital PID controller design technique, examples are provided.