We present a successive microfluidic approach to create and characterize hierarchical catalyst structures consisting of metal-decorated nanoparticles that are assembled into porous microparticles (“supraball” catalysts). First, using a silicon microreactor, platinum nanoparticles with a very narrow size distribution are grown and immobilized uniformly onto iron oxide/silica core–shell nanospheres. The Pt-decorated silica nanospheres are then assembled into uniform, spherical, micron-sized particles by using emulsion templates generated with a microfluidic drop generator. Finally, the assembled supraballs are loaded into a packed-bed microreactor for characterization of the catalytic reactivity. The prepared material showed excellent catalytic activity for the oxidation of aldehyde with only 1 mg of material (containing 50 μg of platinum nanoparticles). After the reactions, all the supraball catalysts are recovered by using the magnetic property of the underlying iron oxide/silica core–shell nanospheres.