This study investigated the spatial and seasonal variations of nutrients, chlorophyll-a (Chl-a), particulate organic carbon, and dissolved oxygen at four stations around Masan Bay from February 2004 to February 2006. Nutrients showed large spatial and seasonal variations in the study area. Dissolved inorganic nitrogen and dissolved silicate concentrations were highest at inner Masan Bay, which was ascribed to the largest inputs of both nutrients from the lands among four stations. Dissolved inorganic phosphorus concentrations are highest at Hangam Bay, where a fertilizer plant caused serious soil pollution with the by-products of phosphate rocks. The extremely high Chl-a concentrations (>60 µg L⁻¹) were observed at inner Masan Bay in July and attributed to the plentiful nutrients, high water stability, and longer residence time at this area during summer. Surface Chl-a concentrations displayed large seasonal variations at all stations, with high values in summer and low values in fall and winter. The lower Chl-a concentrations in fall and winter may be due to strong vertical mixing, which could transport phytoplankton below the euphotic depth, and as a result, constrained phytoplankton growth. Particulate organic carbon–to–Chl-a ratios varied from 59.8 to 121, with large spatial variation and small seasonal change. Hypoxia (<2 mg L⁻¹) was observed at inner and outer Masan and Hangam bays during summer. Outer Masan Bay showed more severe hypoxia than inner Masan Bay in summer 2004 and 2005, which was probably because of the higher organic contents at the bottom sediments and deeper water depth. Despite construction of the Masan and Jinhae wastewater treatment plants, Masan and Hangam bays still received high loading of nutrients, and serious eutrophication was still observed at these bays.