An enhanced spectral response was realized in an AlGaN-based solar-blind ultraviolet (SB-UV) detector using aluminum (Al) nanoparticles (NPs) of 20-60 nm. The peak responsivity of the detector (about 288 nm) with 60 nm Al NPs is more than two times greater than that of a detector without Al NPs under a 5-V bias, reaching 0.288 A/W. To confirm the enhancement mechanism of the Al NPs, extinction spectra were simulated using time-domain and frequency-domain finite-element methods. The calculation results show that the dipole surface plasmon resonance wavelength of the Al NPs is localized near the peak responsivity position of AlGaN-based SB-UV detectors. Thus, the improvement in the detectors can be ascribed to the localized surface plasmon resonance effect of the Al NPs. The localized electric field enhancement and related scattering effect result in the generation of more electron-hole pairs and thus a higher responsivity. In addition, the dark current of AlGaN-based SB-UV detectors does not increase after the deposition of Al nanoparticles. The results presented here is promising for applications of AlGaN-based SB-UV detectors.