SRY-box transcription factor 21 antisense divergent transcript 1 (SOX21-AS1) is a multifaceted long non-coding RNA (lncRNA) that plays diverse roles in both neoplastic conditions and Alzheimer's disease. Its aberrant expression intricately regulates a wide spectrum of cellular processes, spanning from epithelial-mesenchymal transition (EMT), apoptosis, migration, metastasis, and stemness to drug resistance. SOX21-AS1 achieves these effects through its involvement in the competitive endogenous RNA (ceRNA) network, modulation of downstream genes, and regulation of critical pathways, including PI3K/AKT, Hippo, Wnt/β-catenin, and ERK signaling. Of significant clinical relevance, SOX21-AS1 expression has shown robust correlations with various clinical-pathological features. Moreover, it has demonstrated promising prognostic and diagnostic potential across a spectrum of tumors, as evidenced by existing literature and TCGA pan-cancer analyses. In Alzheimer's disease, SOX21-AS1 assumes a distinctive role. It influences neuronal viability, apoptosis, and oxidative stress by interacting with miR-107 and miR-132, and affecting the PI3K/AKT and Wnt signaling pathways. This comprehensive review sheds light on the functions of SOX21-AS1 and the regulated mechanisms underpinning its impact on neoplastic conditions and Alzheimer's disease. It underscores the clinical significance of SOX21-AS1 and positions it as a promising therapeutic target in both the oncological and neurodegenerative domains.