The 1934–35 NHL season was the 18th season of the National Hockey League (NHL). Nine teams each played 48 games. The Ottawa Senators moved to St. Louis and became the St. Louis Eagles. The Montreal Maroons were the Stanley Cup winners as they swept the Toronto Maple Leafs in three games in the Stanley Cup Finals.

In the midst of the Great Depression financial difficulties continued for the Ottawa Senators. The franchise transferred to St. Louis, changing the nickname to the Eagles. The Ottawa organization continued the Senators as a senior amateur team. Despite the new locale the franchise was not profitable in St. Louis either, due in part high travel expenses resulting from still being in the Canadian Division. The Eagles would sell players Syd Howe and Ralph "Scotty" Bowman to Detroit for $50,000 to make ends meet.

Montreal Canadiens owners Leo Dandurand and Joseph Cattarinich sell the team to Ernest Savard and Maurice Forget of the Canadian Arena Company.

The penalty shot, an invention of the old Pacific Coast Hockey Association (PCHA), was introduced in the NHL this season. The puck was placed in a 10-foot circle, 38 feet (12 m) from the goalmouth. The player could shoot while stationary within the circle, or could shoot while moving, as long as the shot was taken within the circle. The goaltender had to be stationary until the puck was shot, and no more than 1 foot (0.30 m) in front of the goal mouth.

Several more teams changed from a single uniform to a light version and dark version. The Detroit Red Wings introduced a white version of their existing uniform, swapping red elements for white elements. The Chicago Black Hawks introduced a new uniform design, and differentiated between versions by using white in the main horizontal stripe and their socks, and using brown in the other version. The New York Americans and Toronto Maple Leafs continued using their two sets of uniforms. The Boston Bruins, Montreal Canadiens, Montreal Maroons, New York Rangers and St. Louis Eagles used only a single uniform design each.

Charlie Conacher decided to play coy this year and Conn Smythe had trouble signing him. With Harvey Jackson out, it looked as though Joe Primeau would be the only member of the Kid line in action for Toronto. However, he did finally sign. Conacher responded with his best season, scoring 36 goals and leading the league in scoring.

A bombshell trade was made with Howie Morenz, Lorne Chabot and Marty Burke going to Chicago for Leroy Goldsworthy, Roger Jenkins, and Lionel Conacher. Although Morenz was not his old self, he did help Chicago, who finished second in the American Division, just falling short of Boston by only one point. The Canadiens then traded Lionel Conacher and Herb Cain to the Maroons for Nels Crutchfield. The trades did not help and the Canadiens lost some fans.

Meanwhile, Tommy Gorman bought a share of the Montreal Maroons from James Strachan and when he picked up Alex Connell, he had another winner.