Proteus mirabilis is an opportunistic pathogen that frequently causes complicated urinary tract infections. Among a wide spectrum of potential virulence factors, outer membrane proteins (OMPs) are critical for bacterial interactions and survival in different environments. In this work, we used a proteomic approach to assess P. mirabilis in vivo OMPs expression compared to in vitro, including iron replete and iron-restricted conditions. Three putative iron receptors, IreA, PMI0842, and PMI2596, were detected both in bacterium grown in vivo and in vitro under iron-restricted conditions. A prophage gene product, PMI1721, was detected only on in vivo growing bacterium, suggesting a potential role yet to be disclosed on the surface of P. mirabilis. Plasminogen, a host protein, was co-purified with OMPs of in vivo grown bacteria, which is in accordance with previous observations and suggests that plasminogen bound to P. mirabilis surface may be associated to virulence as seen in other bacterial pathogens. Western blots using sera of experimentally challenged mice showed that iron-regulated proteins are expressed and highly immunogenic during infection. This work confirms observations made by others for P. mirabilis and reveals details not yet described, suggesting new aspects of the bacterium pathogenesis that remain unknown.