Introduction: Low-level laser therapy (LLLT) is a practical, nonpharmacologic technique for reducing pain. This study evaluated the effect of LLLT on postoperative pain after root canal retreatment (RCR). Methods: This study enrolled patients (N = 36) who required root canal retreatment (RCR) on mandibular molar teeth, presented with periapical lesions with periapical index scores of 2 or 3, and had a pain visual analog scale (VAS) <50 and a percussion pain VAS <50. The participants were divided into 2 groups: (1) patients scheduled for RCR followed by LLLT (n = 18) and (2) patients scheduled for RCR followed by a mock LLLT (placebo) (n = 18). Postoperative pain was assessed using the VAS. Data were collected and statistically analyzed with the chi-square test, the independent sample t test, and the Mann-Whitney U test (P = .05). Results: On the first 4 days, postoperative pain significantly reduced in the LLLT group compared with the placebo group (P < .05). However, no statistically significant differences in postoperative pain were found between the 2 groups after 5 and 7 days (P > .05). The number of patients who needed analgesics was lower in the LLLT group than in the placebo group (P < .05). No patient reported pain during LLLT application. Conclusions: LLLT may reduce postoperative pain after RCR of mandibular molars.