We demonstrate monolithic aluminum gallium arsenide (AlGaAs) optical nanoantennas for enhanced second harmonic generation (SHG) at telecom wavelengths. From measurements on nanocylinders of 400 nm height and varying radius pumped with femtosecond pulses delivered at 1554-nm wavelength, we estimated a peak conversion efficiency exceeding 10−5. Our measurements are in excellent agreement with frequency-domain numerical simulations, revealing the microscopic nature of the SHG process in our nanoresonators.