<p>Macroalgae dominate nutrient dynamics and function as high-value foods for microbial, meio- and macrofaunal communities in coastal ecosystems. Because of this vital role, it is important to clarify the physiological information associated with environmental changes as it reflects their growth potential. To evaluate the effects of the changes in salinity and nutrients, the photosynthetic efficiency of a green macroalga <i>Ulva fasciata</i> from the Daya Bay was tested at a range of salinity (<i>i.e.</i>, 31 to 10 psu) and nitrogen content (<i>i.e.</i>, 5 to 60 µmol L⁻¹). The results showed that cellular chlorophyll <i>a</i> (Chl <i>a</i>), carbohydrate and protein contents of <i>U. fasciata</i> were increased due to reduced salinity, and were decreased by interactive nitrogen enrichment. Within a short culture period (<i>i.e.</i>, 18 h), the reduced salinity decreased the maximum photosynthetic efficiency (rETRmax and Pmax) derived from the rapid light response curve and photosynthetic oxygen evolution rate versus irradiance curve, respectively, as well as the saturation irradiance (<i>E</i>_<i>K</i>). This reducing effect diminished with enlonged cultivation time and reversed to a stimulating effect after 24 h of cultivation. The nitrogen enrichment stimulated the rETRmax and Pmax, as well as the <i>E</i>_<i>K</i>, regardless of salinity, especially within short-term cultivation period (<i>i.e.</i>, &lt; 24 h). In addition, our results indicate that seawater freshening lowers the photosynthetic efficiency of <i>U. fasciata</i> in the short term, which is mitigated by nitrogen enrichment, but stimulates it in the long term, providing insight into how macroalgae thrive in coastal or estuarine waters where salinity and nutrients normally covary strongly.</p>