The Royal Canal (Irish: An Chanáil Ríoga) is a canal originally built for freight and passenger transportation from Dublin to Longford in Ireland. It is one of two canals from Dublin to the River Shannon and was built in direct competition to the Grand Canal. The canal fell into disrepair in the late twentieth century, but much of it has since been restored for navigation. The length of the canal to the River Shannon was reopened on 1 October 2010, but a final spur branch, to Longford Town, remains closed.

In 1755, Thomas Williams and John Cooley made a survey to find a suitable route for a man-made waterway across north Leinster from Dublin to the Shannon. They originally planned to use a series of rivers and lakes, including the Boyne, Blackwater, Deel, Yellow, Camlin and Inny and Lough Derravaragh. A disgruntled director of the Grand Canal Company sought support to build a canal from Dublin to Cloondara, on the Shannon in West County Longford.

Work on this massive project commenced in May 1790 at Cross Guns Bridge, Phibsborough in a westerly direction towards Ashtown. This is commemorated in the plaque beneath the keystone of Ranelagh Bridge.

In Samuel Watson's "The Gentleman's and Citizen's Almanack" for the year 1792, it noted that the Court of Directors for the Royal Canal met at 1 Dawson Street every Tuesday and Thursday at 2 o'clock to discuss matters. At that stage, it was intended that the canal would have branches to (or near) Trim, Kells, Athboy and Castletown-Delvin.

Thomas James Rawson, in his 1807 Statistical Survey of the County of Kildare described the state of completion of the canal at the time, with 3,000 men being in constant employment on the project.

In 1817, twenty-seven years after it began, the canal reached the Shannon. The total cost of construction was £1,421,954. Building was unexpectedly expensive, and the project was ridden with problems; in 1794 the Royal Canal Company was declared bankrupt. The Duke of Leinster, a board member, insisted that the new waterway take in his local town of Maynooth. The builders had to deviate from the planned route; this necessitated the construction of a 'deep sinking' between Blanchardstown and Clonsilla. The diversion also called for the building of the Ryewater Aqueduct, at Leixlip.

In 1796, the canal reached Kilcock and trade commenced. Two passenger boats, the Camden and the Phelan, went into service on 2 December 1796, with a fare of 1s 1d (first class cabin) or 6d (second class cabin) from Dublin to Leixlip, a journey of approximately 20 km. This was much cheaper than the stagecoach at the time, which cost 8s 8d for the same journey. Passengers also had the option to dine on board the canal passenger boat, whereas this was not possible on the coach. This made a slow canal journey such as the 12 hours to Mullingar more manageable.

In the early 1800s, the canal supplied water to Dublin Corporation for its north city water supply, which could make the water level drop on the canal at times, especially during dry summers. To counteract this, a small reservoir named after King George III was opened in the north inner city in 1809, paid for by Dublin Corporation, which was fed from the 1 km-long spur that came off the Royal Canal to meet Broadstone railway station near Phibsborough. The ground alone, covering merely an acre, cost the Corporation £1,052 9s 2d. The reservoir still exists, named the Blessington Street Basin, although the spur has been filled in.