The title compound, [AuCl(C(18)H(21)P)], a monomeric two-coordinate gold(I) complex, has been characterized at 100 K as two distinct monoclinic polymorphs, one from a single crystal, (Is), and one from a pseudo-merohedrally twinned crystal, (It). The molecular structures in the two monoclinic [P2(1)/n for (Is) and P2(1)/c for (It)] polymorphs are similar; however, the packing arrangements in the two lattices differ considerably. The structure of (It) is pseudo-merohedrally twinned by a twofold rotation about the a* axis.