Pluronic/polyethylenimine shell crosslinked nanocapsules with embedded magnetite nanocrystals (PPMCs) were developed for magnetically triggered delivery of siRNA. The positively charged PPMCs formed stable nanosized polyelectrolyte complexes via electrostatic interactions with negatively charged siRNA-polyethylene glycol conjugate (siRNA-s-s-PEG) that was linked via a cleavable disulfide linkage. PPMC/siRNA-s-s-PEG polyelectrolyte complexes were efficiently taken up by cancer cells upon exposure to a magnet, thereby enhancing intracellular uptake and silencing effect of siRNA. The present study suggests that these novel nanomaterials could be potentially utilized for magnetically triggered delivery of various nucleic acid-based therapeutic agents.