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The aim of the study was the serological and structural characterization of the lipopolysaccharide (LPS) O antigen from <i>P. mirabilis</i> Dm55 coming from the urine of a patient from Lodz. The Dm55 LPS was recognized in ELISA only by the O54 antiserum, suggesting a serological distinction of the Dm55 O antigen from all the 84 <i>Proteus</i> LPS serotypes described. The obtained polyclonal rabbit serum against <i>P. mirabilis</i> Dm55 reacted in ELISA and Western blotting with a few LPSs (including O54), but the reactions were weaker than those observed in the homologous system. The LPS of <i>P. mirabilis</i> Dm55 was subjected to mild acid hydrolysis, and the obtained high-molecular-mass O polysaccharide was chemically studied using sugar and methylation analyses, mass spectrometry, and ¹H and ¹³C NMR spectroscopy, including ¹H,¹H NOESY, and ¹H,¹³C HMBC experiments. The Dm55 O unit is a branched three-saccharide, and its linear fragment contains α-Gal<i>p</i>NAc and β-Gal<i>p</i>, whereas α-Glc<i>p</i>NAc occupies a terminal position. The Dm55 OPS shares a disaccharide epitope with the <i>Proteus</i> O54 antigen. Due to the structural differences of the studied O antigen from the other described <i>Proteus</i> O polysaccharides, we propose to classify the <i>P. mirabilis</i> Dm55 strain to a new <i>Proteus</i> O85 serogroup.