Septic arthritis is the most important diagnosis to consider in patients presenting with acute monarticular arthritis. We present the case of an eight-year-old girl who developed a Pseudomonas aeruginosa and Klebsiella pneumoniae septic arthritis of her knee following an injury with a marine sea shell. After treatment with antibiotics and arthroscopic irrigation, she had good functional recovery. We could find no previously reported cases of Pseudomonas aeruginosa or Klebsiella pneumoniae septic arthritis resulting from an injury in a marine environment. The pathogenesis and treatment of septic arthritis and infection following marine injuries are discussed.