Two-dimensional structures of elemental analogues of graphene have received much attention in recent years due to their outstanding properties. Black arsenic is a metastable form of arsenic with an orthorhombic structure similar to black phosphorus. Compared to thermodynamically stable rhombohedral grey arsenic, individual layers are held only by weak van der Waals interactions. Currently black arsenic is available only as an extremely rare naturally occurring mineral arsenolamprite. Herein we demonstrate an easy synthetic route for the synthesis of orthorhombic black arsenic based on the crystallization of amorphous arsenic by mercury vapors. The characterization results confirmed the orthorhombic structure and revealed that it is stabilized by mercuric oxide. As a result, these findings will pave the way for new applications in nanotechnology.