We present here a simple and effective strategy to obtain a series of hydrogel composites using biotite (BT) particles and poly(acrylic acid) (PAA) as the inorganic and organic components, respectively. These hydrogel composites were prepared at room temperature via a self-induced redox system based on ammonium persulfate and Fe(II) arisen from iron-rich BT. After NaOH activation, as-prepared hydrogel composites were used as the adsorbents to remove ammonium (NH4+) from its aqueous solution. The effects of preparation and adsorption conditions on NH4+ removal were investigated. The results show that the adsorption of NH4+ onto as-prepared PAA/BT (30%) is complete within 15 min, with the equilibrium adsorption capacity of 32.87 mg N/g at an initial NH4+ concentration of 100 mg N/L. In addition, the reusability of as-prepared PAA/BT for NH4+ removal was evaluated and the adsorption mechanism of NH4+ onto as-prepared hydrogel composite was proposed.