Bytown is the former name of Ottawa, Ontario, Canada. It was founded on September 26, 1826, incorporated as a town on January 1, 1850, and superseded by the incorporation of the City of Ottawa on January 1, 1855. The founding was marked by a sod turning, and a letter from Governor General Dalhousie which authorized Lieutenant Colonel John By to divide up the town into lots. Bytown came about as a result of the construction of the Rideau Canal and grew largely due to the Ottawa River timber trade. Bytown's first mayor was John Scott, elected in 1847.

Bytown was located where the Rideau Canal meets the Ottawa River and consisted of two parts centred around the canal, Upper Town and Lower Town. Upper Town, situated to the west of the canal, was situated in the area of the current downtown and Parliament Hill. Lower Town was on the east side of the canal where today's Byward Market and general area of Lower Town still exists. The two areas of town were connected over the Rideau Canal by the Sappers Bridge, constructed in 1827.

The town took its name from John By who, as a lieutenant colonel in the British Royal Engineers, was instrumental in the construction of the canal. The name "Bytown" came about, somewhat as a "jocular reference" during a small dinner party of some officers, and it appears on official correspondence dated 1828.

Joseph Bouchette in the summer of 1828 wrote:

The streets are laid out with much regularity, and of a liberal width that will hereafter contribute to the convenience, salubrity and elegance of the place. The number of houses now built is about 150, most of which are constructed of wood; frequently in a style of neatness and taste that reflects great credit upon the Inhabitants. On the elevated banks of the Bay, the Hospital, an extensive stone building, and three Barracks stand conspicuous; nearly on a level with them, and on the eastern side of the Bay, is the residence of Colonel By, Command Royal Engineer at that Station.

Colonel By laid out the streets of Bytown, a pattern that mostly exists today. Wellington Street, Rideau Street, Sussex and Sparks Street were some of the earliest streets in use. Sappers Bridge actually connected Sparks Street to Rideau Street at that time. Nicholas Sparks owned Bytown's land west of the canal, except for the lands north of Wellington, which were considered "Ordnance" lands. The area east of Bank Street to the canal was acquired by the military and not used for houses for around two decades, after which it was returned to him.

The Ottawa River timber trade spurred the growth of Bytown, and it saw an influx of immigrants, and later entrepreneurs hoping to profit from the squared timber that would be floated down the Ottawa River to Quebec. Bytown had seen some trouble in the early days, first with the Shiners' War in 1835 to 1845, and the Stony Monday Riot in 1849.

Some early buildings that still stand had been erected in Bytown. In 1826, Thomas McKay was contracted to build the commissariat building, now the Bytown Museum. McKay also built Rideau Hall (which has since been expanded), and parts of the Union Bridge connecting LeBreton Flats to Hull. Notre-Dame Cathedral Basilica was also built early on in the developing town. The University of Ottawa had its 1846 origins as a college, and it received its present location in 1856.