Metal nanomaterials are playing an increasingly important role in addressing challenges in modern society with respect to energy, catalysis, environment, information and so on. To maximize their performances in different application fields, fine control of their characteristics such as size and size distribution, morphology, composition, structure and surface properties is required. Suitable selection of the synthesis method for metal nanomaterials to achieve such control thus becomes rather important. In addition, the intent to use metal nanomaterials at a large scale puts extra expectations on the synthesis method that should be able to produce metal nanomaterials with high efficiency. Supercritical fluid synthesis of metal nanomaterials appears as a promising way to meet such needs thanks to the unique synthesis environment that speeds up the process but keeps the high controllability and reproducibility. In particular, supercritical fluid synthesis in flows enables continuous synthesis of metal nanomaterials and has high potential to be adapted into an industrial-level production process. This review focuses exclusively on the application of supercritical fluids in the synthesis of non-supported metal nanomaterials in both batch and flow reactors. Advancements in understanding the chemistry processes observed in the synthesis including thermolysis and reductive reactions in various types of fluids under their supercritical conditions are discussed and reviewed, with special attention to identifying the relationship between the properties of metal nanomaterials and the process parameters. Further, the versatility of the chemistry proceeding in supercritical fluids is shown by a few more examples on the synthesis of nanomaterials for applications in cutting edge technologies such as semiconductor nanocrystals, quantum dots, graphenic nanomaterials and metal–organic frameworks. Scaling up the supercritical fluid continuous flow synthesis to levels of pilot plants and even a full industrial plant has been achieved. The latest results and industrial progress in this area are discussed. Given this progress, the evaluation of the environmental impacts of the supercritical fluid flow synthesis becomes rather important. Finally, life cycle assessment (LCA) analysis is introduced as a powerful tool to evaluate the sustainability of chemical synthesis in supercritical fluids, shown by a few examples.