A portable engine is an engine, either a steam engine or an internal combustion engine, that sits in one place while operating (providing power to machinery), but (unlike a stationary engine) is portable and thus can be easily moved from one work site to another. Mounted on wheels or skids, it is either towed to the work site or moves there via self-propulsion.

Portable engines were in common use in industrialised countries from the early 19th through early 20th centuries, during an era when mechanical power transmission was widespread. Before that, most power generation and transmission were by animal, water, wind, or human; after that, a combination of electrification (including rural electrification) and modern vehicles and equipment (such as tractors, trucks, cars, engine-generators, and machines with their engines built in) displaced most use of portable engines. In developing countries today, portable engines still have some use (typically in the form of modern small engines mounted on boards), although the technologies mentioned above increasingly limit their demand there as well. In industrialised countries they are no longer used for commercial purposes, but preserved examples can often be seen at steam fairs driving appropriate equipment for demonstration purposes.

Portable engines during their heyday were typically towed to their work sites by draft horses or oxen, or, in the latter part of that era, motive power including self-propulsion or towing by traction engines, steam tractors, other tractors, or trucks. They were used to drive agricultural machinery (such as threshing machines), milling machinery (such as gristmills, sawmills, and ore mills), pumps and fans (such as in mines and oil wells), and factory line shafts (for machine tools, power hammers, presses, and other machines).

In common with many other areas of steam technology, the initial design and development of portable engines took place in England, with many other countries initially importing British-built equipment rather than developing their own.

Early steam engines were too large and expensive for use on the average farm; however, the first positive evidence of steam power being used to drive a threshing machine was in 1799 in north Yorkshire. The next recorded application was in 1812, when Richard Trevithick designed the first 'semi-portable' stationary steam engine for agricultural use, known as a "barn engine". This was a high-pressure, rotative engine with a Cornish boiler, for Sir Christopher Hawkins of Probus, Cornwall. It was used to drive a corn threshing machine and was much cheaper to run than the horses it replaced. Indeed, it was so successful that it remained in use for nearly 70 years, and has been preserved by the Science Museum in London. Although termed 'semi-portable', as they could be transported and installed without being dismantled, these engines were essentially stationary. They were used to drive such barn machinery as pumps and hammer mills, bone-crushers, chaff and turnip cutters, and fixed and mobile threshing drums.

It was not until about 1839 that the truly portable engine appeared, allowing the application of steam power beyond the confines of the farmyard. William Tuxford of Boston, Lincolnshire started manufacture of an engine built around a locomotive-style boiler with horizontal smoke tubes. A single cylinder and the crankshaft were mounted on top of the boiler, and the whole assembly was mounted on four wheels: the front pair being steerable and fitted with shafts for horse-haulage between jobs. A large flywheel was mounted on the crankshaft, and a stout leather belt was used to transfer the drive to the equipment being driven.

Ransomes built an early portable in 1841 and exhibited it at the Royal Agricultural Society show that year. The next year Ransomes converted the steam engine to self driving, thus making an intermediate step towards the traction engine.

Several Tuxford engines were displayed at the Royal Agricultural Society's Show at Bristol in 1842, and other manufacturers soon joined in, using the basic design of the Tuxford engine as a pattern for the majority of portable engines produced thereafter.