Thymalin is a synthetic dipeptide (pyroGlu-Glu-Asp) originally isolated from calf thymus and developed in the 1970s–1980s at the St. Petersburg Military Medical Academy (Russia) as a standardized polypeptide preparation from thymic tissue. It is classified as an immunocorrector and bioregulatory peptide that acts primarily by normalizing T-cell differentiation, restoring the balance between Th1/Th2 and Treg subpopulations, and stimulating endogenous thymic peptide production (thymulin, thymopoietin, etc.). Extensive clinical and preclinical research, predominantly in Russia and Eastern Europe (over 300 published studies), has investigated thymalin for its ability to restore immune competence in conditions of secondary immunodeficiency, including acute and chronic infections, radiation- or chemotherapy-induced immunosuppression, postoperative recovery, severe burns, and age-related immune decline. Additional studies have explored its geroprotective effects (slowing immunosenescence in elderly patients), adjuvant activity in vaccination of immunocompromised individuals, and potential benefits in autoimmune diseases, oncology (as a supportive agent to reduce myelosuppression), and chronic inflammatory conditions. Although widely used in Russian integrative and anti-aging medicine for decades and approved as a drug in several post-Soviet countries, thymalin remains largely investigational in Western regulatory systems, with most evidence derived from Russian-language clinical trials and limited English-language confirmatory studies.