This paper explores the gas sensing properties of graphene nanolayers decorated with lead halide perovskite (CH 3NH 3PbBr 3) nanocrystals to detect toxic gases such as ammonia (NH 3) and nitrogen dioxide (NO 2). A chemical-sensitive semiconductor film based on graphene has been achieved, being decorated with CH 3NH 3PbBr 3 perovskite (MAPbBr 3) nanocrystals (NCs) synthesized, and characterized by several techniques, such as field emission scanning electron microscopy, transmission electron microscopy and X-ray photoelectron spectroscopy. Reversible responses were obtained towards NO 2 and NH 3 at room temperature, demonstrating an enhanced sensitivity when the graphene is decorated by MAPbBr 3 NCs. Furthermore, the effect of ambient moisture was extensively studied, showing that the use of perovskite NCs in gas sensors can become a promising alternative to other gas sensitive materials, due to the protective character of graphene, resulting from its high hydrophobicity. Besides, a gas sensing mechanism is proposed to understand the effects of MAPbBr 3 sensing properties.