Nasturtium officinale, commonly known as watercress, is rich in various biologically active phenolic compounds, which enable it to be used for the treatment of various chronic diseases. The primary active phytoconstituents in watercress leaves and stems include total polyphenolic compounds, total condensed tannins, and total flavonoids, which were assessed in various extracts, such as aqueous and methanolic, in this study. A variety of in vitro biological activities were assessed, including antioxidant activities such as total antioxidant activity (TAC) and iron-reducing power (IRP), as well as scavenging activities measured against 2,2'-azinobis-3-ethylbenzothiazoline-6-sulfonic acid (ABTS) and 1,1-Diphenyl-2-picryl-hydrazyl (DPPH). Additionally, anti-diabetic activities were measured against the actions of α-amylase and α-glucosidase enzymes, while anti-Alzheimer activities were assessed against the activity of acetylcholinesterase (AChE), and anti-arthritic activities were measured against protein denaturation and the activity of proteinase enzyme. Furthermore, anti-inflammatory activities were evaluated against the actions of cyclooxygenase-1 (COX-1), cyclooxygenase-2 (COX-2), and 5-Lipoxygenase (5-LOX) enzymes. This study concluded that, compared to the other extracts under study, the methanolic extract of watercress exhibited the highest in vitro biological activity. As a result, it may prove to be a useful natural substance in the development of novel treatments for oxidative stress-related disorders.