Introducing exogenous proteins intracellularly presents tremendous chances in scientific research and clinical applications. The effectiveness of this method, however, has been limited by lack of efficient ways to achieve intracellular protein delivery and poor stability of the delivered proteins. Over the years, a variety of nanomaterials have been explored as intracellular protein delivery vectors, including liposomes, polymers, gold nanoparticles, mesoporous silica particles, and carbon nanotubes. Nanomaterials stand out in various protein delivery systems due to various advantages, such as efficient intracellular delivery, long circulation time, and passive tumor targeting. Additionally, chemistry behind these nanomaterials provides readily engineered materials, enabling versatile designs of delivery agents. Intracellular delivery mediated by such nanocarriers achieved varying degrees of success. Different problems associated with these nanocarriers, however, still hamper their real-world applications. Developing new delivery methods or vectors remains essential but challenging. This review surveys the current developments in protein delivery based on synthetic nanocarriers, including liposomes, polymers and inorganic nanocarriers; Prospects for future development of protein delivery nanocarriers are also provided.