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In the present work, we performed immunochemical studies of LPS, especially the O-specific polysaccharide (O-PS) of <i>Aeromonas veronii</i> bv. <i>sobria</i> strain K133, which was isolated from the kidney of carp (<i>Cyprinus carpio</i> L.) during an outbreak of motile aeromonad infection/motile aeromonad septicemia (MAI/MAS) on a Polish fish farm. The structural characterization of the O-PS, which was obtained by mild acid degradation of the LPS, was performed with chemical methods, MALDI-TOF mass spectrometry, and ¹H and ¹³C NMR spectroscopy. It was revealed that the O-PS has a unique composition of a linear tetrasaccharide repeating unit and contains a rarely occurring sugar 2,4-diamino-2,4,6-trideoxy-D-glucose (bacillosamine), which may determine the specificity of the serogroup. Western blotting and ELISA confirmed that <i>A. veronii</i> bv. <i>sobria</i> strain K133 belongs to the new serogroup PGO1, which is one of the most commonly represented immunotypes among carp and trout isolates of <i>Aeromonas</i> sp. in Polish aquacultures. Considering the increase in the MAI/MAS incidences and their impact on freshwater species, also with economic importance, and in the absence of an effective immunoprophylaxis, studies of the <i>Aeromonas</i> O-antigens are relevant in the light of epidemiological data and monitoring emergent pathogens representing unknown antigenic variants and serotypes.