The 1920–21Ottawa Senators season was the club's 36th season of play, fourth season in the NHL, and they were out to defend their 1920 Stanley Cup championship. The club would win the NHL championship and defeat Vancouver in the Stanley Cup Final to win the club's ninth Stanley Cup.
Cy Denneny led the club offensively, scoring 34 goals, which was the 2nd highest total in the NHL, and his 39 points ranked him 3rd in the entire league. Denneny had a 6 goal game against the Hamilton Tigers on March 7.
Frank Nighbor had another great season with 18 goals, and Buck Boucher provided the team toughness, leading the club with 43 penalty minutes.
Clint Benedict also had a great season, earning 14 wins and a league best GAA of 3.08.
The team started the season 8–2–0 and won the 1st half of the season and a spot in the O'Brien Cup finals, but the Senators slumped to a 6–8–0 record in the second half leading to a playoff with the Toronto St. Patricks in a 2 game most goals series.
On December 30, 1920, the Senators were ordered to give up Sprague Cleghorn to Toronto, on the orders of the league. The league ruled that the Senators were over the total salary limit for players and Cleghorn was ordered to report to Toronto. The same day, the league ruled that Punch Broadbent would have to go to Hamilton to complete a trade made before the season, but not completed. Both players refused to report to their new teams. Cleghorn eventually reported to Toronto, but returned to Ottawa in time to play in the playoffs. Broadbent resumed play for Ottawa in the second half of the season.
The series would take place in Vancouver, British Columbia at Denman Arena, and a record breaking crowd of 11,000 people would see the Millionaires win the first game 2–1 to take an early series lead. The Senators would rebound, winning the next 2 before Vancouver would tie the series up at 2 with a win in game 4. Then, for the 2nd straight year, the Senators would win the 5th and deciding game, winning the 1921 Stanley Cup, and became the first team since the 1912 and 1913 Quebec Bulldogs to win back-to-back Stanley Cup Championships.
All of the contests in the series, which went to the full five games, were decided by only one goal. The Millionaires won games one and four with victories of 2–1 and 3–2, respectively. The Senators were victorious in games two and three with 4–3 and 3–2 wins. Then in the fifth game, Jack Darragh scored both of Ottawa's goals in the 2–1 victory to clinch the Cup.
Darragh led the Senators in scoring with five goals during the series, while goaltender Clint Benedict only allowed just 12 goals in the five games, earning a 2.40 goals-against average.
Just like the previous season, the Senators never engraved their name on the Cup for their 1921 championship. It was not until the trophy was redesigned in 1948 that the words "1921 Ottawa Senators" was put onto its then-new collar.