OBJECTIVE To investigate the immediate effect of transcutaneous electrical nerve stimulation (TENS) on spasticity in patients with spinal cord injury. METHODS Randomized controlled trial. METHODS Extended rehabilitation centre. METHODS Eighteen subjects with spinal cord injury and symptoms of spasticity over lower limbs were randomly assigned to receive either 60 minutes of active TENS (0.25 ms, 100 Hz, 15 mA) or 60 minutes of placebo non-electrically stimulated TENS over the common peroneal nerve. METHODS Composite Spasticity Score was used to assess the spasticity level of ankle plantar flexors immediately before and after TENS application. Composite Spasticity Score consisted of Achilles tendon jerks, resistance to full-range passive ankle dorsiflexion and ankle clonus. Between-group statistical differences of reduction of Composite Spasticity Score, Achilles tendon jerks, resistance to full-range passive ankle dorsiflexion and ankle clonus were calculated using the Mann-Whitney test. Within-group statistical differences of Composite Spasticity Score, Achilles tendon jerks, resistance to full-range passive ankle dorsiflexion and ankle clonus were calculated using the Wilcoxon signed ranks test. RESULTS Significant reductions were shown in Composite Spasticity Score by 29.5% (p = 0.017), resistance to full-range passive ankle dorsiflexion by 31.0% (p = 0.024) and ankle clonus by 29.6% (p = 0.023) in the TENS group but these reductions were not found in the placebo TENS group. The between-group differences of both Composite Spasticity Score and resistance to full-range passive ankle dorsiflexion were significant (p = 0.027 and p = 0.024, respectively). CONCLUSIONS This study showed that a single session of TENS could immediately reduce spasticity.