Objective To explore the mechanism by which melanotan-II (MT-II) improves social deficits in a Shank3 gene-deficient autism model. Methods Rats were divided into control and Shank3-deficient model groups (n= 18 per group) treated by microinjection of empty or Shank3-interfering lentivirus, respectively, into the right lateral ventricle of neonatal rats. Shank3-deficient rats were further divided randomly into two groups: Shank3+saline (Sh3-Sal) and Shank3+ MT-II ( Sh3-MT-II) groups ( n= 9 per group). Similarly, control rats were divided into control+saline (V-Sal) and control+MT-II (V-MT-II) groups (n= 9 per group). On day 28, rats in the V-MT-II and Sh3-MT-II groups received intraperitoneal (i. p. ) injections of MT-II (3. 3 mg / kg), while rats in the V-Sal and Sh3-Sal groups received i. p. saline (3. 3 mL / kg). Behavioral changes were assessed using the open-field test, grooming behavior analysis, three-chamber social test, and the Morris water maze test. mRNA and protein expression levels of hypothalamic oxytocin (OXT), OXT receptor (OXTR), and melanocortin receptor 4 ( MC4R) were detected by reverse transcription-polymerase chain reaction and Western blot, respectively. Results In the three-chamber social test, Sh3-Sal group rats showed no significant social preference compared with the time spent with stranger rat 1 (P>0. 05). In contrast, after MT-II intervention, Sh3-MT-II group rats spent significantly longer with stranger rat 2 (P<0. 01). In the Morris water maze test, rats in the Sh3-Sal group exhibited significant learning and memory impairments compared with the V-Sal group (P<0. 05), while MT-II intervention significantly improved the learning and memory performance of the Sh3-MT-II group (P<0. 01). The open field and grooming tests revealed that Sh3-Sal group rats spent significantly longer in the peripheral zone of the open field and exhibited increased grooming behavior compared with the V-Sal group (P<0. 01). However, MT-II did not significantly alter the center time or self-grooming behavior compared with the Sh3-Sal group (P>0. 05). mRNA expression levels of OXT, OXTR, and MC4R were significantly higher in the Sh3-MT-II group than in the Sh3-Sal group (P<0. 05,P<0. 01). Hypothalamic OXT protein expression was significantly increased in the Sh3-MT-II group compared with the Sh3-Sal group (P<0. 05), while hypothalamic SHANK3 protein expression was significantly decreased in both the Sh3-Sal and Sh3-MT-II groups compared with the V-Sal group (P<0. 05,P<0. 01), and protein expression levels of OXTR and MC4R showed no significant changes (P>0. 05). Conclusions The melanocortin receptor agonist MT-II may ameliorate social deficits in Shank3-deficient autistic rats by activating the hypothalamic OXT system. This suggests that targeting the OXT / MC4R pathway could be a potential therapeutic strategy for social deficits in patients with autism spectrum disorder.