Silane coupling agents were successfully incorporated into alkali-activated aluminosilicate geopolymers (GP) through simple mixing into the precursor liquid. This allowed for synthesis of thick coatings with meaningful long-term shear and tensile adhesion to organic polymer substrates. Adhesion of GP to high-impact polystyrene sheets and extruded polystyrene foam was quantitatively evaluated. GP drying shrinkage and coating-substrate thermal expansion mismatch were identified as the major obstacles to adhesion, and these were mitigated by the easy yielding of polystyrene foam. The coupling agent-modified GP experienced decreased splitting strength, but Fourier transform infrared spectroscopy, X-ray diffraction, and scanning electron microscopy were used to verify that the coupling agent addition did not inhibit geopolymerization.