Increasing ultraviolet (UV) radiation is an important environmental stressor affecting marine ecosystems. However, the integrated effects of UV radiation on growth, antioxidant defence mechanisms, and bioactive compound accumulation in marine diatoms remain poorly understood. Chaetoceros calcitrans has attracted considerable attention due to its potential applications in the pharmaceutical, nutraceutical, cosmetic, and aquaculture industries. This study evaluated the effects of photosynthetically active radiation (PAR) combined with UVA (PAR+UVA) and PAR combined with UVA and UVB (PAR+UVA+UVB) on antioxidant enzyme activities and the accumulation of bioactive compounds in C. calcitrans. Exponentially growing cultures were exposed to PAR+UVA or PAR+UVA+UVB for 6, 12, 24, 36, 48, 60, and 72 h. Cultures maintained under PAR without UVA or UVB exposure served as the control. Cell density, optical density, biomass productivity, antioxidant enzyme activities, including superoxide dismutase (SOD), catalase (CAT), and glutathione peroxidase (GPx), as well as bioactive compounds, including fucoxanthin, mycosporine-like amino acids (MAAs), and phenolic compounds, were analysed. SOD and CAT activities reached their highest levels after 72 h under PAR+UVA+UVB treatment, indicating enhanced antioxidant defence in response to prolonged combined UV exposure. In contrast, GPx activity generally decreased following UV treatment and was undetectable after 48 and 60 h under PAR+UVA+UVB treatment. Overall, PAR+UVA+UVB induced greater physiological stress in C. calcitrans than PAR+UVA alone. Fucoxanthin content was significantly higher under PAR+UVA at 24 h (18.71 mg g⁻¹ biomass) and 48 h (17.45 mg g⁻¹ biomass), as well as under PAR+UVA+UVB at 48 h (16.85 mg g⁻¹ biomass), compared with the other treatments. The highest MAA content (porphyra-334) was observed after 48 h of PAR+UVA exposure (256.22 μg g⁻¹ biomass). Conversely, PAR+UVA+UVB exposure for 36 h significantly increased the total phenolic content (750.42 μg g⁻¹ biomass), with gallic acid, 4-hydroxybenzoic acid, catechin, syringic acid, and apigenin identified. These findings suggest that PAR+UVA preferentially promoted the accumulation of photoprotective metabolites, whereas PAR+UVA+UVB elicited stronger antioxidant enzyme responses and phenolic compound production, reflecting greater oxidative stress. This study provides new insights into the antioxidant defence responses and bioactive compound accumulation of C. calcitrans under UV radiation stress. The findings contribute to a better understanding of UV stress tolerance mechanisms in marine diatoms and highlight the potential of controlled UV irradiation as a strategy to enhance the production of high-value microalgal metabolites for biotechnological applications.