J. and G. Rennie was a British engineering company based in Millwall, London, England. They were involved in manufacture of marine engines, and some complete ships, as well as other diverse onshore engineering projects. An association with railway engines is usually attributed to G. and J. Rennie, which may suggest they used a second company to keep the books separate, and there was also George Rennie & Sons, which is associated with the development and patents of the steam disc engine. All three companies appear to have been in existence at the same time.