The crystal structure of entinostat Form B, CHNO, has been solved and refined using laboratory X-ray powder diffraction data, and optimized using density functional techniques. Entinostat crystallizes in space group 2 and the crystal structure consists of inter-locking layers of entinostat mol-ecules parallel to the plane. A strong N-H⋯N hydrogen bond links the mol-ecules into zigzag chains propagating along the -axis direction. The graph set for this pattern is (8). Two N-H⋯O hydrogen bonds link the mol-ecules along the -axis direction. The graph sets for this pattern are (4) and (7).