High‐speed sampling of the voltage waveform on a microstrip transmission line is performed by exploiting the multiphoton photoelectric effect induced by a visible cw mode‐locked laser source. Energy analysis of the electrons emitted from the surface of the strip line is used to infer the emission point potential at the arrival time of the laser pulse. The technique may be applied to measure voltage waveforms on metallization lines of any integrated circuit or electronic device and is capable of picosecond time resolution and millivolt sensitivity.