We examined whether a polymorphism of the PERIOD3 gene (PER3; rs57875989) modulated the sleep promoting effects of melatonin in Delayed Sleep-Wake Phase Disorder (DSWPD). One hundred and four individuals (53 males; 29.4±10.0 years) with DSWPD and a delayed dim light melatonin onset (DLMO) collected buccal swabs for genotyping (PER3 n=43; PER3 5 allele [heterozygous and homozygous] n=60). Participants were randomised to placebo or 0.5mg melatonin taken 1 hour before desired bedtime (or ~ 1.45 h before DLMO), with sleep attempted at desired bedtime (4 weeks; 5-7 nights/week). We assessed sleep (diary and actigraphy), Pittsburgh Sleep Quality Index (PSQI), Insomnia Severity Index (ISI), Patient-Reported Outcomes Measurement Information System (PROMIS: Sleep Disturbance, Sleep-Related Impairment), Sheehan Disability Scale (SDS), and Patient- and Clinician-Global Improvement (PGI-C, CGI-C). Melatonin treatment response on actigraphic sleep onset time did not differ between genotypes. For PER3 carriers, self-reported sleep onset time was advanced by a larger amount and sleep onset latency (SOL) was shorter in melatonin-treated patients compared to those receiving placebo (P=0.008), while actigraphic sleep efficiency in the first third of the sleep episode (SE T1) did not differ. For PER3 5 carriers, actigraphic SOL and SE T1 showed a larger improvement with melatonin (P<0.001). Melatonin improved ISI (P=0.005), PROMIS Sleep Disturbance (P<0.001) and Sleep-Related Impairment (P=0.017), SDS (P=0.019), PGI-C (P=0.028), and CGI-C (P=0.016) in PER3 individuals only. Melatonin did not advance circadian phase. Overall, PER3 DSWPD patients have a greater response to melatonin treatment. PER3 genotyping may therefore improve DSWPD patient outcomes.This article is protected by copyright. All rights reserved.