Two dimensional (2D) crystals with layered magnetism give access to new physical phenomena and technologies. To date, while most 2D magnets exhibit strong out-of-plane anisotropy, intrinsic 2D magnetism with in-plane spin orientation and weak anisotropy remains elusive. We investigate the magnetic order of atomically thin CrCl3 by employing vertical tunneling measurements, which are sensitive to the relative alignment of spins in different layers. We demonstrate that atomically thin CrCl3 is a layered antiferromagnetic insulator with an easy-plane normal to the c-axis; that is, the polarization is in the plane of each layer and has no preferred direction within it. We establish that such intralayer ferromagnetic order and interlayer antiferromagnetic coupling in CrCl3 persist down to the bilayer limit. From the temperature dependence of the magnetoresistance we further obtain an effective magnetic phase diagram for the bilayer. Our work demonstrates a model system for studying the physics of 2D magnetic phase transitions and for making new kinds of vdW spintronic devices.