Abstarct: Water-baxsed drilling fluids are widely used due to their economic and environmental advantages; however, in deep wells and under high-pressure high-temperature (HPHT) conditions, their rheological stability and filtration control deteriorate. This study aimed to synthesize and laboratory-evaluate a poly(o-phenylenediamine)/nanosilica hybrid nanocomposite as an effective additive to improve the performance of bentonite-baxsed water-baxsed drilling fluid at elevated temperatures. The nanocomposite was synthesized via in-situ oxidative polymerization in the presence of nanosilica using ammonium persulfate as an oxidant, and its structure was characterized by FTIR and XRD analyses. Samples containing nanosilica, polymer-containing samples, and hybrid formulations were evaluated for pH, density, rheology, gel strength, filtration under low-pressure low-temperature (LPLT) and high-pressure high-temperature (HPHT) conditions, and filter cake thickness. The results showed that the presence of the hybrid composite simultaneously improved rheological properties and filtration control. Adding the hybrid nanocomposite to the bentonite-baxsed water-baxsed drilling fluid improved rheological stability, enhanced gel strength, reduced fluid loss, and formed a thinner and less permeable filter cake at high temperature. By reducing the consumption of other fluid loss control agents, decreasing mud loss, reducing the frequency of mud treatment, and lowering the risk of pipe sticking, this additive can reduce drilling costs and, through better filtration control and wellbore stability, increase well safety under HPHT conditions. Therefore, the synthesized nanocomposite can be considered an efficient option for water-baxsed drilling fluids in deep wells.