Accessory and atypical muscles of the upper limb are common, whereas symptomatic variations presenting with carpal tunnel syndrome (CTS) are rare. A rare unilateral accessory transverse carpal muscle located palmar to the transverse carpal ligament is described. The accessory muscle, associated with CTS clinical manifestations in a 38-year-old Greek male worker, can be quite problematic during CTS operation. The current case emphasizes the importance of meticulous knowledge of the variable anatomy of the carpal tunnel (CT) area, since the accessory muscles may be accompanied by ectopic motor branches and a high risk of iatrogenic injury. Recognition and careful evaluation of accessory muscles may enhance a surgeon's ability to carry out a safer and successful CT approach.