Tatton-Brown-Rahman syndrome (TBRS) is characterized by overgrowth, cognitive deficiency, and distinctive facial features resulting from germline DNMT3A variants. This report describes a four-year-old female diagnosed with TBRS due to a de novo and novel heterozygous DNMT3A variant, NM_022552.5:c.1627G>C:p.(Gly543Arg). Alongside typical TBRS features, she had a history of hospitalization for immune thrombocytopenic purpura (ITP) at five months old. While ITP is clinically diagnosed and has multifactorial origins, studies have demonstrated its autoimmune and genetic components. DNMT3A protein, responsible for DNA methylation, regulates various cellular processes, including hematopoiesis and autoimmunity. It has been reported that ITP patients exhibit decreased expression of DNMT3A, and specific variants linked to decreased platelet counts have been identified in a murine model for TBRS. Additionally, some case reports have been described with multiple cytopenias and thrombocytopenia without hematologic malignancy. In conclusion, this report emphasizes for the first time the occurrence of ITP in a TBRS patient and suggests that autoimmune and hematologic disorders may need to be considered in the follow-up of these patients. However, further evidence is required to establish a direct correlation.