Single-molecule detection via surface-enhanced Raman scattering (SERS) has raised great interest over the past decade. The usual approach toward this goal is to harness the strong surface plasmon resonance of light with complex metallic nanostructures, such as particle aggregates, two-particle gaps, sharp tips, or particles with sharp apexes. Here we propose another route toward the goal by introducing gain medium into single metal nanoparticles with simple geometry. Our calculations show that cubic gold nanobox particles that contain a gain material within the core can create an extremely high enhancement factor of local field intensity larger than 10⁸ and a SERS enhancement factor on the order of 10¹⁶−10¹⁷.