The S100A12 protein was validated as a biomarker of health status in porcine saliva samples using a semi-quantitative approach based on Western blotting in four healthy and sixteen diseased animals, and in four animals with severe respiratory disease during three days of antibiotic therapy. Afterwards, a non-competitive sandwich immunoassay was then developed, validated, and used to quantify S100A12 in clinical porcine samples, using 14 healthy and 25 diseased pigs. Finally, the S100A12 concentrations in the saliva of ten pigs with respiratory disease were monitored during antibiotic therapy. Diseased animals showed higher concentrations of S100A12 than healthy animals, and the high concentrations of S100A12 in pigs with respiratory distress were reduced after antimicrobial therapy. The assay developed showed good precision and accuracy, as well as a low limit of detection of 3.19 ng/mL. It was possible to store saliva samples at -20 °C, or even at 4 °C, for two weeks before analysis without losing the validity of the results. The concentrations of S100A12 observed in serum and saliva samples showed a moderately positive association with a correlation coefficient of 0.48. The concentrations of the new validated biomarker S100A12 are highly associated with the novel salivary biomarker of inflammation, adenosine deaminase, and moderately to highly associated with the total oxidant status. The results reported in this study provide a new way of evaluating inflammatory diseases in pigs using saliva samples, which should be further explored for disease prevention and monitoring in the field.