A magnetic adsorbent was synthesized by γ-aminopropyltriethoxysilane (APTES) modification of Fe3O4 particles using a two-step process. Scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and vibration sample magnetometry were used to characterize the obtained magnetic adsorbent. EDS and XPS showed that APTES polymer was successfully introduced onto the as-prepared Fe3O4/APTES particle surfaces. The saturation magnetization of the magnetic adsorbent was around 65 emu g−1, which indicated that the dye can be removed fast and efficiently from aqueous solution with an external magnetic field. The maximum adsorption capacities of Fe3O4/APTES for C.I. Reactive Red 228 (RR 228) and Congo Red (CR) were 51.4 and 118.8 mg g−1, respectively. The adsorption of C.I. Reactive Red 228 (RR 228) and Congo Red (CR) on Fe3O4/APTES particles corresponded well to the Langmuir model and the Freundlich model, respectively. The adsorption processes for RR 228 and CR followed the pseudo-second-order model. The Boyd's film-diffusion model showed that film diffusion also played a major role in the studied adsorption processes for both dyes. Thermodynamic study indicated that both of the adsorption processes of the two dyes are spontaneous exothermic.