This triple-arm, double-blind, randomized clinical trial investigated the effect of multisession prefrontal transcranial direct current stimulation (tDCS) on social functioning in individuals with autism spectrum disorder (ASD). A total of 105 individuals (age 14-21 years) diagnosed with ASD were randomized into the active-tDCS, sham-tDCS, and waitlist control groups. The experimental group received 20 min of 1.5 mA tDCS stimulation (cathode: F3; anode: Fp2) for 10 days with concurrent computerized cognitive remediation training. Changes in overall social functioning, social communication, and restricted, repetitive behaviors (RRB) were measured by the Social Responsiveness Scale-2nd edition (SRS-2). Two-level hierarchical linear mixed-model analysis showed significant group*time interactions for SRS-2 total [F(2, 107.09) = 7.82; p = 0.001] and RRB [F(2, 90.26) = 5.62; p = 0.005] T-scores. When compared with the waitlist control group, the changes in scores were only significant in active-tDCS (SRS-2 total T-score p < 0.001, d = 0.61; SRS-2 RRB T-score p = 0.002, d = 0.88), but not sham-tDCS (SRS-2 total T-score p < 0.12, d = 0.30; SRS-2 RRB T-score p = 0.54, d = 0.17) group. Multiple sessions of prefrontal tDCS coupled with cognitive training is a safe and effective treatment for improving social functioning in people with ASD. Trial Registration: ClinicalTrials.gov (ID: NCT03814083; URL: https://clinicaltrials.gov/ct2/show/NCT03814083).