2D materials are considered as excellent candidates for next-generation electronic and optoelectronic devices. However, the corresponding systems with both an appropriate direct band gap and high carrier mobility are urgently required. Here, a new 2D semiconductor, monolayer RhTeCl, is investigated based on first-principles calculations. Monolayer RhTeCl possesses a direct band gap of 2.16 eV, with a high electron mobility up to 1.5 × 104 cm2 V-1 s-1. Thus, monolayer RhTeCl double-gated metal-oxide-semiconductor field-effect transistors (MOSFETs) with a 6 nm gate length are simulated by quantum transport methods. The 6 nm monolayer RhTeCl n-MOSFET displays a steep sub-kT/q switching characteristic and a high on/off ratio (106), which demonstrates a superior gate control. Therefore, these promising semiconductor characteristics and device performances of 2D RhTeCl provide new opportunities for novel low power ultra-scaled devices.