Bulk MgB₂ with SiO₂ nanoparticles added has been synthesized using a simple solid-state reaction route. The lattice constant in the c direction increases with additive content due to a small amount of Si being doped into the lattice of the MgB₂; however, T_c is almost fixed at 37.2 K. The addition of SiO₂ nanoparticles also improves the J_c–H and H_irr–T characteristics of MgB₂ when the additive content is lower than 7%. At 20 K and 1 T, J_c for the sample with 7% additive content reaches 2.5 × 10⁵ A cm⁻². Microstructural analysis reveals that a high density of MgSi₂ nanoparticles (10–50 nm) exists inside the MgB₂ grains, leading to the formation of a nanocomposite superconductor.