In this work, the continuous determination of hexazinone in water was carried out by developing an online solid phase extraction by flow injection coupled with spectrophotometric detection. Variables related to hydrodynamic conditions were optimized by a full factorial design 3^2, of which the results were analyzed through an analysis of variance. Under the proposed optimal conditions, the principal figures of merit were a working range between 0.50 to 7.00 μg mL^-1 of hexazinone; a precision of 4.6% expressed as variance coefficient; a limit of detection of 0.05 μg mL^-1, and a limit of determination of 0.16 μg mL^-1. Univariant calibrations based on the height or area of transitorial signals were compared to identify the best conditions for quantification. Samples of well and sea water were analyzed, obtaining satisfactory results in all cases in terms of precision and accuracy.