Optical second‐harmonic generation at 1.06 &mgr;m was demonstrated in the cubic (4¯3m) and orthorhombic (mm2) phases of Cr‐Cl, Cu‐Cl, Fe‐I, and Ni‐Br boracite. The discontinuous change in the second‐harmonic intensity is consistent with the known first‐order nature of the ferroelectric phase transition. Both phases of boracites are suggested to be non‐phase‐matchable for second‐harmonic generation and for cubic boracites; indirect methods result in nonlinear optical coefficients (d123=d213=d312) in the range (0.20 to 0.26) ×10−9esu. in the range (0.20 to 0.26)×10−9esu.