The absolute signs and magnitudes of all the optical second‐harmonic generation coefficientsdijfor ferroelectric guanidinium aluminum sulfate hexahydrate (GASH) have been obtained by using single‐domain crystals and a 1.06‐&mgr;m Nd laser. It is found that in esu unitsd22= − 0.25 × 10−9,d31=d15= + 0.020 × 10−9, andd33= + 0.047 × 10−9, where the absolute directions are defined in the usual manner. The magnitudes ofd22andd31are consistent with values estimated from data previously reported by others. The vector part of the &dgr; form of thedtensor is &dgr;&ngr;= 2&dgr;31+ &dgr;33= + 1.2 × 10−7esu. The value &dgr;&ngr;/Ps= + 3.4 × 10−7and is about −5 times the value characteristic of a number of oxide‐type ferroelectrics. Upon polarization reversal all thedijcoefficients change sign with respect to a fixed coordinate system, which is consistent with structure data and symmetry considerations.