The orthorhombic compounds BaMF4(M=Mn, Fe, Co, or Ni) have recently been found to undergo antiferromagnetic transitions at low temperatures. Susceptibility measurements on BaNiF4reveal that a broad maximum occurs at about 150°K similar to that observed in a number of layer‐type structures. As the temperature is lowered, &khgr;∥ alongbdecreases rapidly, while &khgr;⊥ goes through a shallow minimum around 50°K. At 77°K, no magnetic order can be detected in powder neutron diffraction patterns of BaNiF4, but at 4.2°K there are magnetic peaks which can be indexed on a unit cell doubled along thebandcdirections (A21amorientation). The magnetic structure consists of puckered (010) sheets within which the moments are coupled antiparallel to neighboring moments about 4‐Å apart. However, the crystal structure is such that the net interaction between adjacent sheets is zero, and ordering in thebdirection depends upon interactions between second nearest sheets 14‐Å apart. This is very similar to the situation inK2NiF4and pronounced two‐dimensional behavior is therefore to be expected. The moments are directed along thebaxis and the ordered moment is about 2.0 &mgr;Bper Ni ion. The magnetic symmetry is actually monoclinic, and the simplest magnetic space group isP2a21.