Introduction: Septic arthritis is a dreadful condition affecting joints which warrants an emergency arthrotomy in tandem with debridement followed by intravenous antibiotics during the peri-operative period. It generally affects larger joints, namely the hip, knee, shoulder, ankle and elbow. Serratia marcescens causing septic arthritis is uncommon in an immunocompetent individual and the available literature on the same, is limited.  Case presentation: A 73-year-old female diabetic patient, had presented to the casualty with swelling in the right knee with local rise of temperature and limited movements for five days. She had undergone left total knee arthroplasty four months ago with no postoperative complications and was ambulating well. The radiographs did not show signs of gross abnormality. Synovial fluid aspirated from the affected knee was found to have Serratia marcescens infection localized to the joint, as the blood culture was negative for the same organism. Left knee arthrotomy with debridement was done along with polyethylene liner exchange and local antibiotic delivery via calcium sulphate crystals mixed with vancomycin, tobramycin and gentamicin. Following this, she was given intravenous antibiotics based on the culture and sensitivity   report.  Conclusion: Septic arthritis of the knee joint caused by Serratia marcescens is a rare occurrence, especially following total knee arthroplasty. Hence special attention had to be given for surgical planning and treatment regimen. Knee effusion with painful limitaion of movements is a pathognomonic sign of a septic knee. Initial antibiotic cover remains the mainstay of the treatment but in an acute setting, knee arthrotomy with debridement and application of local antibiotic delivery gave excellent results.