The aims of the current study were to use the leaves of Syzygium jambos (syn. Eugenia jambos L.) for the biosynthesis of silver (AgNPs) and gold (AuNPs) nanoparticles and to evaluate their in vitro antioxidant, antimicrobial, cytotoxic activities as well as their total phenolic content(TPC).The antioxidant activity was evaluated qualitatively using dot-blot and DPPH staining, and quantitatively via phosphomolybdenum assay. Also, the in vitro antimicrobial activity was evaluated via disc agar plate method against five pathogenic microbial strains including Gram +ve and Gram -ve bacteria & fungi. The preliminary cytotoxic activity was evaluated via brine shrimp lethality test (BSLT), and total phenolic content (TPC) was estimated via Folin-Ciocalteu’s assay. Silver (AgNPs) and gold (AuNPs) were synthesized and characterized via UV-vis absorbance spectroscopy, transmission electron microscopy (TEM), and X-ray diffraction (XRD) analyses. The results indicated that AgNPs and AuNPs can be synthesized using Syzygium jambos leaf extract. The transmission electron microscopy (TEM) analysis showed that the sizes of the synthesized AgNps ranged from 6-23nm, and the synthesized AuNPs also exhibited an average size of 6-23nm. Moreover, the results revealed that TPC of the tested extracts was ranged from 548.85 to 123.30 mg gallic acid equivalent (GAE)/ g dry extract. The total antioxidant capacity (TAC) was ranged from 643.90 to 147.96mg ascorbic acid equivalent (AAE)/g dry extract. Furthermore, there is a promising antimicrobial activity against four strains viz., Pseudomonas aeruginosa with inhibition zones from 9 to 15.5mm; Staphylococcus aureus with inhibition zones from 9 to 14.5mm; Methicillin-resistant Staphylococcus aureus with inhibition zones from 8.5 to 14.5mm and Candida albicans with inhibition zones from 9.5 to 13.5mm; respectively for 85% MeOH, defatted 85% MeOH, pet. ether, CH2Cl2, EtOAc, n-BuOH, H2O, and 85% MeOH of flower part), while the cytotoxic results showed LC50 values ranged from 50.11 to 446.68 μg/ ml. In, conclusion the leaves of Syzygium jambos showed a noticeable antioxidant, antimicrobial & cytotoxic activities and the ability to produce AgNPs and AuNPs.