Background: Spasticity following traumatic brain injury (TBI) is one of the most significant barriers of returning patients to their normal life. Spasticity caused by TBI does not have a specific or definitive treatment, and the clinical effect of pharmacologic treatments has not been significant. Methods: In this single-arm study, we evaluated 15 patients. For each patient with spasticity, treatment with oral baclofen 25 mg was started three times a day as a part of standard therapy. After 48 hours, if the spasticity did not decrease by at least one score in the Modified Tardieu or Ashworth scales, lidocaine 0.5% was administered as a continuous intranasal infusion. The initial dose of lidocaine was 1 mg/min, which was gradually increased to 2 mg/min. Spasticity and the frequency of spasms were assessed by Ashworth and modified tardieu scales (MTS) and Spasm Frequency Score (SFS), respectively. Heart rate (HR), respiratory rate (RR), mean arterial blood pressure (MAP), Richmond Agitation-Sedation Scale (RASS), Glasgow Coma Scale (GCS), and arterial oxygen saturation (SPo2) of patients were recorded during nine days of treatment. All data were analyzed by SPSS version 21. P-value less than 0.05 was considered as statistically significant. Results: Out of 15 participants in this study, 13 (86.7%) were male, and 2 (13.3%) were female (mean age: 29.26 ± 12.5 years). There were no significant differences in Ashworth Scale, Modified Tradieu Scale, RASS Score, GCS Score, MAP, SPo2 percentage, HR, RR, and the number of spasms per day between the time of initiation of treatment and the second day of baclofen treatment (P > 0.05). Evaluation of spasticity using Ashworth scale on the first and last days of lidocaine treatment showed a significant decrease in the mean spasticity (3.46 ± 0.51 and 1.46 ± 0.91, respectively; P < 0.001). Spasticity assessment using the MTS showed a significant reduction in the mean of the last day of treatment compared to the mean of the first day of treatment (3.6 ± 0.5 and 1.26 ± 0.51, respectively; P < 0.001). This decrease was also seen in the mean of the last day of treatment compared to the first day in SFS (13.3 ± 3.88 and 3.8 ± 0.51, respectively; P < 0.001). Comparison of HR, RR, MAP, RASS, GCS, and SPo2 on the first and last days of treatment did not show any statistical differences. Conclusions: Although continuous intranasal treatment with lidocaine can be effective in spasm reduction of patients with TBI, further studies with larger sample sizes and longer follow-up periods are required.