Expansion microscopy (ExM) enables super-resolution imaging by physically enlarging biological samples. While ExM has been successfully applied to study the intracellular microtubule cytoskeleton, reliable probes for visualizing actin fibers remain limited. Here, we present HAK-actin, an engineered actin probe compatible with ultrastructure expansion microscopy (U-ExM). We show that HAK-actin delivers robust and uniform actin staining across diverse systems, including human cells, microbial eukaryotes, and mouse retinal tissue. This tool provides a simple, versatile, and reproducible solution for actin cytoskeleton visualization, addressing a critical need in cell biology.