BACKGROUND Several methods have been used for fibrinogen determination in dogs, but to the authors' knowledge, methods based on ammonium sulfate precipitation have not yet been reported in this species. OBJECTIVE The aim of this study was to develop and validate an automated method based on ammonium sulfate precipitation for canine fibrinogen determination. METHODS A reagent containing ammonium sulfate, sodium chloride, and K2 EDTA was used to precipitate fibrinogen at a final ammonium sulfate concentration of 0.57 M and final turbidity was measured on a Cobas Mira Plus autoanalyzer. Analytic validation included imprecision, accuracy, comparison with reference method (Clauss), limits of detection and quantification, and the evaluation of the influence of different anticoagulants. For diagnostic validation, fibrinogen was determined in a group of Beagle dogs before and after neutering, and in dogs affected by diseases known to produce low fibrinogen plasma concentration, such as liver insufficiency, disseminated intravascular coagulation, and protein-losing enteropathy. RESULTS Low imprecison (<4%), excellent recovery (>90%), and low bias (0.092 g/L) with respect to Clauss method indicated a high reproducibility and accuracy. Limits of detection and quantification were 0.01 and 0.22 g/L, respectively. The method was applicable in plasma samples anticoagulated with EDTA, heparin, or sodium citrate. The fibrinogen concentration in Beagle dogs after neutering was increased, and decreased in animals with disseminated intravascular coagulation, liver insufficiency, or gastrointestinal protein loss. CONCLUSIONS The automated method validated in this study represents a rapid, cheap, and easy protocol to quantify canine fibrinogen in routine practice.