Coreflood experiments were conducted to evaluate the impact of the initial rock wettability on oil recovery during a tertiary oil recovery process using hydrophilic silica-based nanofluids at reservoir temperatures. An adopted scale of reservoir wettabilities (water-, intermediate-, and oil-wet systems) is used for the core plugs, which were prepared by aging processes. The relationships among temperature, initial wettability, and an additional oil recovery nanofluid flooding process were investigated. The results showed that the initial wettability affects oil recovery performance and showed a greater effect at higher temperatures, as represented by the reservoir temperature. An extended postflush nanoflooding was performed to evaluate incremental oil recovery, and this cycle shows great potential for field applications. By evaluating the contact angle and interfacial tension, it was found that wettability alteration plays a more dominant role in the oil displacement mechanism via nano-EOR. These results reveal a potential way to use silica-based nanofluid for enhanced oil recovery purposes for a wide range of reservoir wettabilities at a given reservoir temperature.