Nanotechnology is a crucial field that is being widely researched in various fields. This innovative study investigates the synthesis of cerium oxide nanoparticles (CeO<inf>2</inf>NPs) using an extract from the plant Alternanthera sessilis and its various biological applications. Several methods, such as UV–Vis spectroscopy, FTIR, XRD, FESEM, HR-TEM, and DLS analysis, were used to confirm the nature of the synthesized CeO<inf>2</inf>NPs. In this study result, the UV–Vis spectroscopy maximum absorption peak at 326 nm proved that CeO<inf>2</inf>NPs were present. FTIR analysis identified several functional groups, while XRD confirmed their crystal structure. It was seen in FESEM and HR-TEM images that CeO<inf>2</inf>NPs were mostly spherical and oval, and their average size was 29 nm. CeO<inf>2</inf>NPs exhibited impressive antioxidant properties and exhibited significant antibacterial activity against E. coli (25.04 ± 1.45 mm) and S. aureus (23.97 ± 1.29 mm). CeO<inf>2</inf>NPs showed significant anti-inflammatory effects by inhibiting COX-1 (80.91 ± 1.22%) and COX-2 (71.58 ± 1.16%). CeO<inf>2</inf>NPs showed strong enzyme-inhibitory activity against α-amylase (70.46 ± 1.37%), α-glucosidase (79.58 ± 1.37%), acetylcholinesterase (68.92 ± 1.39%), and butylcholinesterase (73.47 ± 1.53%). Cytotoxicity assays showed that CeO<inf>2</inf>NPs significantly reduced cell viability by 28.59 ± 0.39% in HepG2 cancer cells. In conclusion, this study shows that the green-synthesized CeO<inf>2</inf>NPs is a non-toxic, cost-effective and safe method, making it a very promising alternative for various biological applications. © 2025 Elsevier B.V., All rights reserved.