Elemental 2D materials exhibit intriguing heat transport and phononic properties. Here we have investigated the lattice thermal conductivity of newly proposed arsenene, the 2D honeycomb structure of arsenic, using ab initio calculations. Solving the Boltzmann transport equation for phonons, we predict a highly anisotropic thermal conductivity, of 30.4 and 7.8 W/mK along the zigzag and armchair directions, respectively at room temperature. Our calculations reveal that phonons with mean free paths between 20 nm and 1 µm provide the main contribution to the large thermal conductivity in the zig-zag direction, mean free paths of phonons contributing to heat transport in the armchair directions range between 20 and 100 nm. The obtained low and anisotropic thermal conductivity, and feasibility of synthesis, in addition to other reports on high electron mobility, make arsenene a promising material for a variety of applications, including thermal management and thermoelectric devices. PACS numbers:The discovery of graphene as a stable atomically thin material has led to extensive investigation of similar 2D systems. Its properties such as high electron mobility 1 , and very high thermal conductivity 2-5 make graphene very appealing for applications in electronics, for packaging and thermal management 6-11 . The successful isolation of single-layer graphene fostered the search for further ultra-thin 2D structures, such as silicene, germanene, phosphorene, and transition metal dichalcogenides, e.g. MoS 2 and WS 2 12,13 . These materials are now considered for various practical usages due to their distinguished properties stemming from their low dimensionality. Thermal transport in 2D materials has recently attracted the attention of the scientific community, as anomalous heat conduction has been predicted to occur in systems with reduced dimensionality 14 . Phononic properties and thermal conductivity vary significantly from one 2D system to another [15][16][17][18] . For example, silicene has a buckled structure and a lower thermal conductivity 19,20 compared to graphene 12,21,22 .2D structures of arsenic and phosphorous have been recently investigated [23][24][25][26][27] . Arsenic and phosphorus are in the 5th group of the periodic table and both have different allotropes. Black phosphorus is a layered allotrope of phosphorus similar to graphite, and the stability of its single layer form, named phosphorene has been probed both theoretically and experimentally 13,28 . Gray arsenic is one of the most stable allotropes of arsenic with a buckled layered structure 27,29 . In addition, arsenic has an orthorhombic phase (puckered) similar to black phosphorus 23,25,26 , and its monolayer is called arsenene (see Fig. 1). Experimental observations have shown that gray arsenic undergoes a structural phase transition to the orthorhombic precursor of arsenene at temperatures of about T = 370 K 30 . As a monolayer arsenene has a direct band gap as opposed to the multilayer allotrope, which exhibits an indirect band gap of the order...