The red tide in Hongsha Bay caused by Skeletonema costatum (S. costatum) from April 27 to May 4, 2006, was monitored in this study. The dynamic variety of environmental factors, including chlorophyll a (chl a), temperature, salinity, pH, dissolved oxygen, chemical oxygen demand, nitrate, ammonium, nitrite, phosphate (PO₄³⁻), silicate, and iron (Fe), was observed and analyzed during the red tide in Hongsha Bay for the first time. The results indicated that the concentration of dissolved inorganic nitrogen was high (26.34 μmol/L) in Hongsha Bay. Because of the heavy rainfall from April 13 to 15, a large input of nutrients surged into the bay, causing an increase in the concentration of various nutrients, especially PO₄³⁻, which showed an obvious increase (from 0.72 to 1.45 μmol/L). The abundant nutrients provided fundamental nutrient supply for the rapid proliferation of S. costatum. Three critical environmental factors, including water temperature, and PO₄³⁻ and Fe concentration, played an important role in this red tide. Water temperature had a significant positive correlation with chl a. The water temperature shift was one of the critical environmental factors affecting the S. costatum red tide in Hongsha Bay. With the occurrence of the red tide, the concentration of PO₄³⁻ rapidly decreased. Inorganic PO₄³⁻ was rapidly depleted at the blooming stage, causing the red tide to gradually dissipate. Phosphate was the limiting factor of S. costatum proliferation in this red tide. Iron was also a factor. Salinity shift had little effect on the growth of S. costatum.