The Ottawa Rough Riders were a Canadian Football League team based in Ottawa, Ontario, founded on September 19, 1876. Formerly one of the oldest and longest-lived professional sports teams in North America, the Rough Riders won the Grey Cup championship nine times. Their most dominant era was the 1960s and 1970s, in which they won five Grey Cups. The team's fortunes waned in the 1980s and 1990s, and they ultimately ceased operations following the 1996 season. Five years later, a new CFL team known as the Ottawa Renegades was founded, though they suspended operations in 2006. The Ottawa Redblacks, which own the Rough Riders and Renegades intellectual properties, joined the league in 2014.

The Ottawa Football Club was organized on Wednesday, September 20, 1876, where they won the first game they played on September 23 against the Aylmer Club at Jacques Cartier Square. The team's colours were cerise, grey, and navy blue. The club adopted the name Ottawa Rough Riders on Friday, September 9, 1898, and changed its team colours to red and black. Since then, red and black have been Ottawa's traditional sporting colours. Although in later years the name was said to derive from logging (or more specifically, the logdrivers who guided timber down the rivers), the team based its colours on Teddy Roosevelt's regiment in the Spanish–American War, which, with the date of the renaming, suggests that the name also comes from the war. The team changed its nickname to Ottawa Senators from 1925 to 1930.

Ottawa's first Canadian championship came in 1898. The Ottawa Football Club transferred from the Quebec Union to the Ontario League that season. The Riders defeated the Hamilton Tigers 15–8 for the Ontario championship, then defeated Toronto Varsity, the Intercollegiate champions 7–3 and defeated Ottawa College 11–1 to win the Canadian championship. In those days, Ottawa athletes played in multiple sports and the Riders had athletes famous in other sports, such as Harvey Pulford and Frank McGee. The Riders and Ottawa College were the Canadian champions for the next several years, with the Riders defeating Brockville 17–10 in 1900, and defeating Ottawa College 5–0 in 1902, College being the 1901 Canadian champions. The Riders moved back to the Quebec Union, winning the 1903 Quebec championship, in a year where there was no playoff for the Canadian title. In 1905, Ottawa won the Quebec title, only to lose to the Toronto Varsity team 11–9 in the Canadian championship.

The club absorbed the Ottawa St. Pats when the Riders helped found the Interprovincial Rugby Football Union in 1907. The Riders won the IRFU championship in 1909 over the Hamilton Tigers, but lost in the Canadian final in Toronto to Toronto Varsity. The Riders declined and became uncompetitive during the 1910s, attributed to the First World War, and the lure of salaries in professional ice hockey meaning athletes chose hockey over football in Ottawa.

During the decline of the Riders, another Ottawa team, Ottawa St. Brigids, was on an ascent. St. Brigids, which played in the Ottawa City league, and later the Ontario league, was developing top talent. In 1923, St. Brigids and the Riders merged, with St. Brigids manager Jim McCaffery becoming the manager of the Riders. McCaffery was a member of the Riders executive for several decades. The team won the Grey Cup in 1925 and 1926, a time when they were known as the Ottawa Senators. In 1925, Ottawa defeated three-time defending champion Queen's in the Eastern semi-final. Ottawa then defeated Winnipeg 24–1 in the championship, held in Ottawa, and defeated Toronto Varsity 10–7 in Toronto in 1926. The team was led by top players such as Eddie Emerson, Joe Tubman, Joe Miller, Jess Ketchum, Jack Pritchard, Harold Starr and Don Young.

The Riders went back into a decline after the championships. Again, another Ottawa team, the Ottawa Rangers, was developing talent and enjoying success, winning the Quebec title. The Riders absorbed the Rangers in 1933, getting Rangers stars Andy Tommy, Arnie Morrison and "Fat Quinn'. That same year the Riders added more talent, bringing in American imports "Windy" O'Neil and Lorne Johnson. In 1935, the Riders added Roy Berry who was mysterious about his origins. The Riders defeated the Toronto Argonauts in the final two games of the Big Four schedule to deny Toronto the Big Four championship. On November 19, 1935, Toronto's The Mail and Empire accused the Riders of using an illegal player in those matches. Editor Edwin Allen stated "Roy Berry" was an alias for Bohn Hilliard, a Texas Longhorns footballer who had played semi-professional baseball. In February 1936, Amateur Athletic Union of Canada president W. A. Fry announced the suspension of nine members of the Ottawa team due to the Roy Berry incident.

In 1936, the Riders won the Big Four title defeating the Hamilton Tigers 3–2. The team progressed to the Eastern final against the Sarnia Imperials. The Imperials won the game 26–20 in a frozen battle held at Toronto's Varsity Stadium. Since there was no western challenge that year, the Imperials became Canadian champions. The highlight of Rough Rider Joe Zelikovitz's football career came in the Big Four game in Hamilton against the Hamilton Tigers on October 15, 1938, when he set the Big Four record with seven interceptions, a record that still stands unofficially compared to the CFL. The Riders next won the Big Four and Eastern title in 1939, but lost to the Winnipeg Blue Bombers 8–7 in the Grey Cup game, held in Ottawa. In 1940, the Riders won the Big Four and Eastern titles, defeating Toronto Balmy Beach. The win over Balmy Beach carried the Canadian title, as the west refused the Canadian Rugby Union code.

The Big Four went out of existence during the Second World War, but the Riders were able to field a club in the Eastern Rugby Football Union, along with Balmy Beach, Montreal and the Argonauts. The Riders won the 1942 ERFU title over the Argonauts, but again lost to the Blue Bombers in the Canadian final, 18–16 at Varsity Stadium.