Phototoxicity induced by antibiotics is a real problem in health care. The discontinuation of antibiotic therapy due to a phototoxic reaction can lead to the development of resistant strains. Fluoroquinolones are widely used antibiotics that exhibit phototoxic activity under UVA radiation. The purpose of the study was to examine the redox status of human dermal fibroblasts exposed to UVA radiation and treated with lomefloxacin, the most phototoxic fluoroquinolone. Lomefloxacin alone was found to have an antiproliferative activity on fibroblasts by affecting the cell cycle. In addition, the drug caused a redox imbalance associated with the decreased expression of catalase and glutathione peroxidase. UVA radiation increased the drug cytotoxicity and oxidative stress induced by lomefloxacin. The decrease in cell viability was accompanied by a high level of reactive oxygen species and extensive changes in the antioxidant levels. The revealed data indicate that the phototoxic action of lomefloxacin results from both increased reactive oxygen species production and an impaired antioxidant defense system. Considering all of the findings, it can be concluded that lomefloxacin-induced phototoxic reactions are caused by an oxidoreductive imbalance in skin cells.