© 2015 AIP Publishing LLC. We experimentally demonstrate the use of the magnetic-field-dependence of highly spatially confined, GHz-frequency ferromagnetic resonances for the detection of magnetic nanoparticles using an anti-dot-based magnonic crystal. The stray magnetic fields of nanoparticles within the anti-dots modify nano-confined ferromagnetic resonances in the surrounding periodically nanopatterned magnonic crystal, generating easily measurable resonance peak shifts. The shifts are comparable to the resonance linewidths for high anti-dot filling fractions with their signs and magnitudes dependent upon the mode localization, consistent with micromagnetic simulation results. This is an encouraging result for the development of frequency-based nanoparticle detectors for nano-scale biosensing.