Optical second harmonic generation, thanks to its coherent nature, is a suitable signal for interferometric measurements, such as digital holography: a well-established imaging technique that allows recovering complex diffraction wavefields from which it is possible to extract both amplitude- contrast and quantitative phase images. Here, we report on a new, multi-functional form of microscopy: second harmonic generation digital holographic microscopy. As a proof of concept, we have investigated the second harmonic signal generated at the glass/air interface of a microscope slide under focused femtosecond laser illumination and propose for the first time a representation and interpretation of the recovered phase. In this simple, yet very educative case study, we observe that the second harmonic is generated by the axial component of the incident field polarization