This article presents a method for estimating the grid impedance with application to islanding detection and online tuning of a current controller that regulates the grid current in a grid-tied inverter with LCL filter. The presented solution requires a low computational effort when implemented in a converter that is commanded by a finite control set (FCS) current controller, such as a FCS model predictive controller (MPC), which selects and holds during a sampling period a switching state from the set of all discrete switching states of the power converter. Similarly to a noninvasive technique, the presented estimation method does not perform any signal injection nor perturbs the grid. Nonetheless, it provides an accurate estimation with a high resolution because it employs the large variable-pulse-width output voltage of the voltage source converter (VSC) and the measured grid current in order to perform the estimation of the grid impedance.