This study was implemented to determine the seasonal variations in the nutrient concentration and water quality of the Gulf of Gemlik. Samples were collected seasonally at totally 11 stations at 7 different depths (0.5-50 m) between July 2010 and April 2011. 6 of these stations were littoral stations and 5 were offshore stations. During the study, temperature, salinity and dissolved oxygen levels of the seawater ranged between 8.7-28.3 degrees C, 14.98-36.59 ppt and 2.63-17.18 mg l(-1), respectively. Nitrate+nitrite-N (0.01-21.14 mu g-at l(-1)), ammonium-N (0.02-1.93 mu g-at l(-1)), phosphate-P (0.1-5.1 mu g-at l(-1)) and silicate-Si (0.08-33.06 mu g-at l(-1)) concentrations were measured. Also chlorophyll a values ranged between 0.00-10.64 mu g l(-1). The lowest and highest level of N:P ratio were obtained to be 0.16 and 28.92. Trophic Index (TRIX) values, which were calculated to describe water quality, were between 0.91 and 4.92. According to Bray-Curtis similarity index, the stations which had vertical depths had a similarity of 94%, while littoral stations were approximately 85% similar to each other. Trophic Index values indicated that this region has the lowest trophic level but the highest quality status.