Nitrite and nitrate, which are the essential nutrients for marine phytoplankton growth, play an key role in nitrogen biogeochemical transformations, and exert a major influence on the primary productivity of marine ecosystems. It is necessary to develop accurate, sensitive and automated analytical methods to determine nitrite and nitrate in waters. The widely used spectrophotometric method for determination of nitrite is the Griess assay, which is based on the diazotization reaction of nitrite with sulfanilamide and N-1-naphthylethylenediamine dihydrochloride and the formation of an azo dye under acidic conditions. The determination of nitrate is based on the reduction of nitrate to nitrite and subsequent spectrophotometric determination of the nitrite with Griess assay. The copperized cadmium column is the most commonly used for nitrate reduction to nitrite, due to its high and stable reduction efficiency. However, cadmium is highly toxic, and potentially harmful to human health, so is the generated waste. In this study, an automatic reserve flow injection method using vanadium (III) as a reduction agent for simultaneous determination of nitrite and nitrate in estuarine and coastal waters was reported for the first time. Vanadium (III) chloride was used to replace the toxic cadmium for reducing nitrate to nitrite. The experimental parameters were optimized based on a univariate experimental design. The salinity effect of estuarine and coastal waters was carefully investigated. With the optimized conditions, the detection limit of the proposed method was 0.06 µmol·L-1 and 0.13 µmol·L-1, and the linearity was up to 20 µmol·L-1 and 80 µmol·L-1 for nitrite and nitrate detection, respectively. The relative standard deviations were below 1.5% (n=7). The recovery of spiked estuarine and coastal water samples varied from 100.0±2.5% to 107.5±2.5% for nitrite and 90.7±0.3% to 98.0±1.0% for nitrate. The sample throughput was about 15 h-1. The analytical results obtained with the proposed method showed good agreement with those using a reference method. The proposed method has been successfully applied to analyze the nitrite and nitrate in estuarine and coastal water samples.