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 o C, 14.98-36.59 ppt and 2.63-17.18 mg l -1 , respectively. Nitrate+nitrite-N (0.01-21.14 µg-at l -1), ammonium-N (0.02-1.93 µg-at l -1), phosphate-P (0.1-5.1 µg-at l -1) and silicate-Si (0.08-33.06 µg-at l -1) concentrations were measured. Also chlorophyll a values ranged between 0.00-10.64 µg 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 calcu-lated to describe water quality, were between 0.91 and 4.92. According to Bray-Curtis similarity index, the sta-tions 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.