Creatine Transporter Deficiency (CTD) is a rare X-linked disorder caused by loss-of-function of the SLC6A8 gene. CTD clinical manifestation follows brain creatine (Cr) deficiency and predominantly leads to neurological symptoms such as intellectual disability, language disturbance, psycho-motor impairment, autistic-like behavior, and seizures. We used a murine model to probe whether perinatal adeno-associated virus (AAV)-mediated expression of a functional SLC6A8 transgene under the control of a ubiquitous promoter could restore creatine transporter (CRT) expression and brain Cr level, thus preventing the onset of CTD pathological phenotypes. Our results show that early intracerebral expression of human SLC6A8 (hSLC6A8) increases brain Cr levels in SLC6A8 knockout (KO) mice, partially ameliorating brain connectivity alterations and behavioral impairment. However, the same strategy did not improve cognitive functions in KO mice, while inducing deterioration of behavioral performance in WT mice. We hypothesize that the detrimental effects of hSLC6A8 delivery result from dramatic overexpression outside of the context of endogenous CRT production. Thus, we are developing a second-generation gene therapy cassette to maximize the beneficial effects and mitigate potential safety concerns of the treatment. As an alternative to the use of non-native promoters for expressing SLC6A8, we are exploring a portion of its endogenous regulatory sequence. Our in vitro data suggest that this approach reduces transgene expression, better mimicking CRT physiological levels. Altogether, our results provide proof-of-concept evidence that gene therapy holds potential as a disease-modifying treatment for CTD and suggest that further steps in vector engineering to finely tune CRT expression are crucial for optimizing efficacy.