In light of their easy processing, light weight and mechanical flexibility, ferroelectric molecular crystal with large spontaneous polarization (P-s) is highly desired for many advanced applications. Herein, we report the first theoretical study of two-dimensional (2D) ferroelectric molecular crystals via ab initio calculations. Specifically, we show that diisopropylammonium bromide (DIPAB) based 2D ferroelectric monolayer molecular crystal with large in-plane P-s of similar to 1.5 x 10(-6) mu C cm(-1) can be achieved by slicing the bulk DIPAB along a specific plane while keeping the space group unchanged. The important roles of hydrogen bonds are also identified. Ab initio molecular dynamics simulations indicate that, with the support of a graphene substrate, the ferroelectric order of 2D DIPAB monolayer can be retained at room temperature. Lastly, we show that several other diisopropylammonium halide molecular crystals can also be used to achieve 2D all-organic ferroelectric monolayer singular molecular crystal with large in-plane P-s.