[Purpose] This placebo-controlled study investigated the therapeutic effects of low-level laser treatment (LLLT) on the transverse carpal ligament in carpal tunnel syndrome (CTS). [Subjects and Methods] Idiopathic CTS patients were recruited and were randomly assigned to two groups. The laser group (n = 45) received laser treatment (10 Hz, 60 mW, 9.7 J/cm², 830 nm), and the placebo group (n = 42) received sham laser treatment. The visual analog scale (VAS), Boston Questionnaire scale, neurological symptoms and nerve conduction study (NCS) were conducted before, immediately after and 5 weeks after the treatment. [Results] After LLLT, there was statistically significant decrease in VAS in the laser group (p<0.05). Especially, the effect of LLLT on pain alleviation in the mild CTS group continued after five weeks. Regarding the Boston Questionnaire scale neurological symptoms and NCS, only mild CTS patients in the laser group showed statistically significant improvements after treatment (p<0.05). [Conclusions] LLLT with 830 nm diode lasers on the transverse carpal ligament had verifiable therapeutic effects for mild CTS patients.