The covalent functionalization of GaN and AlN surfaces with organosilanes is demonstrated. Both octadecyltrimethoxysilane and aminopropyltriethoxysilane form self-assembled monolayers on hydroxylated GaN and AlN surfaces, confirmed by x-ray photoelectron spectroscopy and atomic force microscopy. The monolayer thickness on GaN was determined to $2.5\pm 0.2\mathrm{nm}$ by x-ray reflectivity. Temperature-programmed desorption measurements reveal a desorption enthalpy of $240\mathrm{kJ}∕\mathrm{mol}$ . The realization of micropatterned self-assembled monolayers and the hybridization of deoxyribonucleic acid molecules on biofunctionalized GaN surfaces are shown.