Magnesium ammonium phosphate was formed in flow-through microchannels of silica monoliths using two different methods to fabricate materials that show efficient ammonia adsorption performance from wastewater with low hydraulic resistance. Magnesium ammonium phosphate crystals in these materials release ammonia when heated at 378 K, yielding primarily magnesium hydrogen phosphate. When this material was used for ammonia removal from an aqueous solution containing 100 ppm ammonia in a flow system, the material readily removed ammonia, decreasing the ammonia concentration to 25 ppm. The material can be reactivated by the same procedure and used again for ammonia removal. Hydrodynamic resistance through the lengths of the materials depend on the shape of the immobilized crystals, showing that needle-like crystals are more effective to cause less resistance than plate-like particles. The material containing needle-like crystals causes only approximately one-eighth of the hydraulic resistance that a packed column consisted of spherical particles with a typical bed porosity of 0.5 does. Thus, these results demonstrate the high applicability of the material for ammonia removal from wastewater in a continuous process.