The Grohovo landslide is located on the northeastern slope in the central Rječina River Valley, and the river mouth is located in the center of Rijeka City, Croatia. It is the largest active landslide along the Croatian coast of the Adriatic Sea. This complex retrogressive landslide was induced by an earthquake that occurred in Rijeka City at the end of the 18th century and was reactivated in December 1996. The Grohovo landslide poses a natural hazard to Rijeka City, and so it was necessary to perform a three-dimensional (3D) slope stability assessment. Slope deposits in the Grohovo landslide are mainly a mixture of clayey silt that was formed by weathered Paleogene flysch rock and fragments of limestone originating from the cliffs on the top of the slope. The basal failure surface is located at the contact between the slope deposits and the flysch bedrock. Based on borehole data, geological mapping, and the reconstruction of geological cross sections through the slope, this study interpolated the 3D shape and position of the sliding surface using a modified inverse distance weighted interpolation method. The entire Grohovo landslide and 12 separate sliding blocks, which have the same 3D sliding surface, were analyzed using a modified 3D slope stability analysis based on a simplified force-equilibrium scheme.