BACKGROUND: Vibrio plantisponsor was first isolated in 2011 from mangrove-associated wild rice roots in India. Human infections caused by this species have not been reported previously. CASE PRESENTATION: An 80-year-old man with prostate adenocarcinoma was admitted to the hospital with fever and chills. Initial diagnostic challenges arose when standard methods, including matrix-assisted laser desorption/ionization time-of-flight mass spectrometry and the VITEK 2 system, failed to definitively identify the gram-negative bacterium. Polymerase chain reaction (PCR) analysis of 16S ribosomal RNA suggested the organism was either V. plantisponsor or Vibrio diazotrophicus. Subsequent PCR and sequencing of the gyrB gene revealed higher sequence similarity to V. plantisponsor. The patient initially responded well to intravenous ceftazidime, but ultimately succumbed to terminal cancer. CONCLUSION: This case highlights the limitations of conventional diagnostic technologies and the importance of advanced molecular tools for identifying rare pathogens. V. plantisponsor should be considered a potential opportunistic pathogen in immunocompromised hosts.