To detect indentation or low-velocity impact induced damage in foam-core sandwich structures, a fiber-optic distributed strain monitoring system is applied to the structures. An optical fiber is embedded into the sandwich structures and the strains are measured using a Rayleigh scattering based monitoring system which offers high resolution. Indentation loading tests with sandwich beam and panel structures are conducted to verify the monitoring ability of the system and compared with numerical analysis predictions. The monitoring system could measure the strain distribution with very high spatial resolution and allowed for observation of the strain formation throughout the whole indentation event. The detectability of the damage in panel structures with regards to the location and density of the optical fiber sensor network is also discussed. The used system could accurately detect slightest changes in residual strains indicating barely visible damage and thus allowed for good estimation of the location and size of damage in the core of the sandwich structures.