Ultraviolet-screening compounds from the cyanobacterium Lyngbya sp. CU2555 were partially characterized and investigated for their induction by UV radiation, stability under different abiotic factors, and free radical scavenging activity. Based on the high-performance liquid chromatography coupled with diode array detector and ion trap liquid chromatography/mass spectrometry analysis, the compounds were identified as palythine (UVλ max: 319 nm; m/z: 245), asterina (UVλ max: 330 nm; m/z: 289), scytonemin (UVλ max: 384 nm; mw: 544), and reduced scytonemin (UVλ max: 384 nm; m/z: 547). This is the first report for the occurrence of palythine, asterina, and an unknown mycosporine-like amino acids (MAA), M-312 (UVλ max: 312 ± 1 nm), in addition to scytonemin and reduced scytonemin in Lyngbya strains studied so far. Induction of MAAs and scytonemin was significantly more prominent upon exposure to UV-A + UV-B radiation. Both MAAs and scytonemin were highly resistant to some physicochemical factors such as UV-B, heat, and a strong oxidizing agent and exhibited strong antioxidant activity. These results indicate that the studied cyanobacterium may protect itself from deleterious short-wavelength radiation by synthesizing photoprotective compounds in response to harmful UV radiation.