Li[TCNE] (TCNE = tetracyanoethylene) magnetically orders as a weak ferromagnet (canted antiferromagnet) below 21.0 +/- 0.1 K, as observed from the bifurcation of the field-cooled and zero-field-cooled magnetizations, as well as remnant magnetization. The structure, determined ab initio from synchrotron X-ray powder diffraction data, consists of a planar mu(4)-[TCNE](center dot-) bound to four tetrahedral Li+ ions. The structure consists of two interpenetrating diamondoid sublattices, with closest interlattice distances of 3.43 and 3.48 angstrom. At 5 K this magnetic state is characterized by a coercivity of similar to 30 Oe, a remnant magnetization of 10 emu center dot Oe/mol, and a canting angle of 0.5 degrees.