Antioxidants isolated from plants have attracted the interest of clinicians and common people to be used for systemic uses rather than synthetic antioxidants because of their active role in maintaining human health with minimal side effects. Alnus nitida (Spach) Endl. is an important medicinal plant native to western Himalaya and is widely distributed throughout Pakistan. The present study evaluates the phytochemical composition of this plant using HPLC along with the total content of phenolics and flavonoids. The antioxidant activities were determined following the Brand William assay. The methanolic extract (Met. Ext) of leaves, stem bark, seeds, and roots of A. nitida were used to scavenge synthetic free radicals such as DPPH and ABTS. From HPLC fingerprinting of the A. nitida selected portion, six possible phytochemicals were confirmed. Among the identified phytochemicals, there are six compounds (malic acid, chlorogenic acid, epigallocatechin gallate, quercetin, ellagic acid and pyrogallol) in the leaves of A. nitida, three (epigallocatechin gallate, ellagic acid, and pyrogallol) in the stem bark, six in the seeds (malic acid, vitamin C, epigallocatechin gallate, quercetin, ellagic acid, and pyrogallol), and five (malic acid, epigallocatechin gallate, quercetin, and ellagic acid) in root. Comparatively, the highest antioxidant potentials were recorded for the leaves extract (IC50 of 340 and 645 µg/mL against DPPH and ABTS, respectively). The percentages of inhibition were compared with the positive control ascorbic acid, which produced an IC50 value of 60 μg/mL each against the free radicals DPPH and ABTS. The highest phenolics (43.81 mg GAE/g sample) were found in the roots, while the highest flavonoid contents (53.25 mg QE/g sample) were in the leaves. It was assumed that observed antioxidant potentials of the tested plant might be due to their phytochemicals confirmed through HPLC, and thus, this plant may be a valuable candidate in treating oxidative stress and related disorders. However, further investigations are needed to isolate responsible components in pure from. Furthermore, toxicological effects in in vivo animal models are also needed to confirm the results observed in this study.