We report the first case of early postoperative infection after a medial hemiarthroplasty of the knee with a customized ConforMIS iForma™ interpositional device. The infection was treated successfully by revision surgery with implant removal and antibiotic therapy. Despite the additional diagnosis of rheumatoid arthritis that did not affect the treated knee, the preservation of bony and ligamentous structures enabled a successful re-implantation of another iForma™ implant 9 months later with good clinical results at follow-up examination 1 year postoperatively. This is very much in contrast to the extensive and complex revision surgery, with significant bone loss, in patients with infected unicompartmental or total knee arthroplasties. The iForma™ device may be an alternative treatment option in early and moderate unicompartmental arthritis of the knee, with easy revision with the same type of implant in the rare case of infection.