Abstract: L-Theanine (LTA), a distinctive free amino acid in tea, is the major contributor to the sweet and fresh taste in tea infusions. It exhibits a range of biological activities, as a novel food ingredient. This study screens the Web of Science Core Collection and China National Knowledge Infrastructure databases of LTA-related literature from 2014 to 2024 and utilizes Citespace and VOSviewer for a visual analysis. Keyword density and contribution mapping indicate that LTA research predominantly centers on tea varieties, stress responses, and the biological activities of LTA. Consequently, this review synthesizes the factors influencing LTA content in tea and elucidates the mechanisms underlying LTA's biological activities, including antioxidant, heat stress resistance, antidepressant, metabolic regulation, anti-cancer, immune regulation, gut microbiota modulation, neuroprotective, hepatoprotective and intestinal protective effects. The results suggest that LTA mediates its biological functions primarily through the modulation of signaling pathways such as nuclear factor kappa B (NF-κB), c-Jun N-terminal kinase (JNK), signal transducer and activator of transcription (STAT) and mitogen-activated protein kinase (MAPK). Furthermore, the current applications of LTA in the food industry are summarized. This review aims to provide direction for LTA research and development and offer a reference for the formulation of LTA-based functional foods.