Based on density functional theory calculations and Monte Carlo simulations, we propose UI3 monolayer as a candidate of f-electron two-dimensional magnet. The UI3 monolayer has two kinds of crystal structures, which are rectangular and hexagonal, respectively. Our calculations show that both of them have ferromagnetic ground state, and the hexagonal structure is more stable than the rectangular one. Notably, the hexagonal UI3 monolayer has a 110 K Curie temperature. Furthermore, the large magnetic anisotropy, strong exchange interactions, and heavy spin-orbit couplings enable the hexagonal UI3 monolayer to be an promising candidate material for future spintronic applications.