Domain boundaries in lead phosphate, P b 3 ( P O 4 ) 2 , are investigated by using second harmonic generation (SHG) microscopy. It is revealed that they are polar while the bulk is a purely ferroelastic, centrosymmetric material. P b 3 ( P O 4 ) 2 contains W and W ′ walls with different orientations. The SHG microscopy shows that both W and W ′ walls are polar, in striking contrast to the bulk, where the macroscopic polarity is forbidden by symmetry. The strongest polarity is observed in the W walls with a local symmetry of monoclinic m . The W ′ walls are also monoclinic but with symmetry 2. This material is hence a new candidate for domain-boundary-induced ferroelectricity while the bulk remains fully inactive. An application of small stress to a W wall easily changes the boundary configuration without changing the type of domain boundary; i.e., the W-wall nature is conserved. While P b 3 ( P O 4 ) 2 was so far suggested as a device material for acousto-optic switchers, the present study adds the possibility that light scattering can be modified also by electric fields.