Background: Oenanthe fistulosa L. (Apiaceae) is often associated with damp soils. Its underground parts and the young leaves are mainly cooked with other vegetables. Objective: The aim of the current work was to investigate the chemical profile of dichloromethane (DCM), Ethyl Acetate (EA) and n-butanol (BuOH) fractions of O. fistulosa through analysis of 37 phytochemicals by LC-MS/MS and to evaluate their biological activities such as antioxidant, anticholinesterase and antityrosinase for the first time. Methods: Analysis of 37 phytochemicals was performed by Liquid Chromatography-Mass Spectrometry (LC-MS/MS). Antioxidant activity was evaluated using five in vitro assays, while anticholinesterase and anti-tyrosinase activities were performed using Ellman and Dopachrome methods, respectively. Results: The number of phenolic compounds detected in DCM, EA and BuOH fractions was found to be 9, 15, and 12; respectively. More specifically, 9 phenolic acids were detected and among them, chlorogenic, tr-ferulic and p-coumaric acids were the most abundant. While 8 flavonoids were detected and apigetrin, rutin, and quercitrin were the most abundant. In addition, 3 non-phenolic organic acids (quinic, malic and fumaric acids) were detected in large quantities. Furthermore, the tested plant fractions demonstrated a noteworthy and strong antioxidant action. The plant displayed very strong action against Acetylcholinesterase (AChE) and butyrylcholinesterase (BChE) enzymes; and BuOH fraction was the most potent one. Finally, BuOH and DCM fractions showed good tyrosinase inhibitory activity. Conclusion: According to the obtained results, O. fistulosa might be a promising candidate for the alleviation of oxidative stress, neurodegenerative (such as Alzheimer’s disease) and hyperpigmentation disorders.