Two-dimensional (2D) nanostructures are gaining tremendous interests due to the fascinating physical, chemical, electrical, and optical properties. Recent advances in 2D nanomaterials synthesis have contributed to optimization of various parameters such as physical dimension and chemical structure for specific applications. In particular, development of high performance gas sensors is gaining vast importance for real-time and on-site environmental monitoring by detection of hazardous chemical species. In this review, we comprehensively report recent achievements of 2D nanostructured materials for chemiresistive-type gas sensors. Firstly, the basic sensing mechanism is described based on charge transfer behavior between gas species and 2D nanomaterials. Secondly, diverse synthesis strategies and characteristic gas sensing properties of 2D nanostructures such as graphene, metal oxides, transition metal dichalcogenides (TMDs), metal organic frameworks (MOFs), phosphorus, and MXenes are presented. In addition, recent trends in synthesis of 2D heterostructures by integrating two different types of 2D nanomaterials and their gas sensing properties are discussed. Finally, this review provides perspectives and future research directions for gas sensor technology using various 2D nanomaterials. Graphical Abstract