<p>As immunoregulatory cells, regulatory T cells (Tregs) play pivotal roles in maintaining immune tolerance and preventing autoimmunity. However, Tregs exhibit distinct functions across different diseases. In cancer, Tregs are most likely to suppress antitumor immune responses and promote tumor immune evasion, whereas in inflammatory diseases, functionally competent Tregs mitigate excessive immune activation and facilitate tissue repair. Notably, dysfunctional Tregs lead to persistent inflammation and progression to chronic disease. Therefore, targeting Tregs has emerged as an attractive immunotherapeutic strategy for both cancer and inflammatory disorders. Recent studies have shown that Tregs exhibit instability and plasticity under specific conditions, allowing them to shift between functional and dysfunctional states. A comprehensive understanding of the dynamic changes in Tregs and their regulatory mechanisms in diverse pathological contexts is highly important. In this review, we summarize the dual roles of Tregs in cancer and various inflammatory diseases. We explore the signaling pathways and molecular mechanisms underlying their biological characteristics, with a particular focus on how microenvironmental cues shape Treg behavior. Additionally, we discuss recent advances in Treg-targeted therapies in these disease contexts. Overall, this review greatly advances our understanding of the roles of Tregs in cancer and inflammation and helps inform the development of more precise and effective therapeutic strategies.</p>