Ubiquitin-like proteins play important roles in diverse biological processes. In this study, we present an unexpected finding that a ubiquitin-like small archaeal modifier protein (SAMP2) from Haloferax volcanii adopts two distinct states under low ionic condition. One of these is similar to the β-grasp structure conserved in ubiquitin-like proteins from eukaryotes; the other is disordered, like prokaryotic ubiquitin-like protein, Pup. Furthermore, our study reveals that the conformation of SAMP2 is dependent on ionic strength. With the increase of ion concentration, SAMP2 undergoes a conformational conversion from disorder to order, indicating that the ordered conformation is the functional form of SAMP2 under the physiological condition of H. volcanii.