Single crystals of KNiPO4 were synthesized in a KCl flux. The crystals have the orthorhombic space group Pna21 at room temp. with a = 8.6333(5), b = 9.2565(5) and c = 4.9064(9)[.ANG.]. Ferroelectric 180 Deg domains, parallel to [001] were revealed by chemical etching. Magnetization measurements along all three orthorhombic axis, made on a SQUID magnetometer, show a magnetic phase transition to the antiferromagnetic state at TN(25 +- 0.5) K with a weak ferromagnetic behavior. All possible coeffs. of the linear magnetoelec. effect have been tested, only a12 and a21 were found to be non zero. These components of the linear magnetoelec. effect are consistent with magnetic point group mm2 which was also found by neutron diffraction studies. However, this magnetic point group does not allow weak ferromagnetism, which is also obsd. The pyroelec. nature of this compd. has also been put into evidence by measuring a change of the spontaneous polarization DPs = 0.61 [mC cm-2] between 4 and 300 K. The total pyroelectric coefficient at 300 K is p3 = 2.75 x 10-5 [Cm-2] K-1].